Battery Life
The best purchase, in game or out of game, you can make is a battery case or pack, and this is true even if you do not play much Pokémon Go. We all forget to fully charge our phone sometimes. I personally use an Alpatronix iPhone 6 Plus battery case. It lasts more than three hours even through nonstop Pokémon Go use, so it survives an afternoon of raiding or an entire Community Day with room to spare.
Some people have one, but the Pokémon Go Plus is not cheap and has a significant limitation. The device only throws one Pokéball before you run. It is not particularly effective for anything difficult to catch. I believe that buying a battery case or pack, or certain things from the Shop mentioned below, will be a better bang for your buck.
Where to Play
The Pokémon Go radar at this point should be fairly self-explanatory. The places with the most Pokéstops will usually have the most in quantity and the most variety. You can usually figure out what the most popular places are in your area, or you can search “Community Day [your area]” and you will figure out where to go. They are usually parks, universities, or shopping malls.
For example, the best places to play in Tucson are Reid Park due to its variety (map link) and 4th Avenue for its dense Pokéstops (map link). Know your park hours and be careful in high crime areas after dark.
Join a Facebook group for your area. There will be people who have a nest list, which changes every few weeks. This will tell you where certain Pokémon spawn regularly. The mediocre places only have one or two Pokéstops and a Pokémon might pop up every hour. Those are usually not worth the time and effort.
The moderate places have several Pokéstops. They may require some patience between spawns, but they are doable. The best places are those with a dozen plus Pokéstops where that Pokémon spawns in clusters. Some places will be walking parks, while others will be driving parks where you can stop and park at various spots to catch Pokémon. There are some Pokémon that will never be on a nest list, and others that can only be hatched by egg or evolved from a base form.
Weather patterns will affect which Pokémon are more readily available and whether they get a weather boost in terms of CP.
Driving for Pokéstops
To quickly get a lot of items and gifts, find a street nearby with lots of Pokéstops where you can drive at or slower than 20 miles per hour, preferably a quieter street at that time of day and preferably with parking spots. Have a passenger spin Pokéstops for you and themselves.
Gyms
Gyms do not matter as much as they used to, but you can put Pokémon in gyms of your color and take down gyms. If you keep Pokémon in gyms, they get you Pokécoins, which you can then use on buying various items. Certain Pokémon which are walls, such as Blissey, will hold gyms for a longer period of time.
Gyms are also Pokéstops that can be spun. You just have to open the gym first before spinning the Pokéstop.
Daily Events
Catch a Pokémon, spin a Pokéstop, complete one field research task, and exchange one gift with a friend. You can have field research tasks carry over.
On the seventh consecutive day of spinning a Pokéstop and completing a field research task, you get a special evolution item and a chance at a rare Pokémon, respectively. Until July 2018, this rare Pokémon was always a legendary. You rack up experience points faster by doing all four things.
Items
Pokémon storage space and bag space are the most useful, and permanent. Other good items depending on what you are going for include lucky eggs for experience points, super incubators to hatch more eggs faster, and premium raid passes if you have a good group going who wants to do multiple legendary raids.
I personally put the most Pokécoins in Pokémon storage space, bag space, and premium raid passes, as I am okay with getting experience slower and hatching eggs slower since I already play a lot.
Lucky eggs and Stardust typically last 30 minutes. Use them during Community Day, legendary raids, or before reaching Ultra Friend status with a friend to double your experience points in that time period and quickly rack up experience points. Incense and Lures are not as useful anymore, but can be used in similar circumstances.
Stardust is also useful for upgrading Pokémon. Take a look at the most powerful Pokémon in Pokémon Go to see which ones are best to use it on. Stardust is also used for trading. I do not recommend upgrading Pokémon with stardust until you are at least Level 30, when you can max out possible CP on Pokémon you see in the wild.
TMs, which you can win through raids, can be useful for changing up the moves to something more powerful. In terms of trading, wait until you are Ultra Friends, which takes 90 days, with a friend before trading to dramatically drop the amount of Stardust required. Trades can only be local and within a certain distance.
Use rare candies on Pokémon that are 5km buddy walking distance. Walk buddy Pokémon that are 1km buddy walking distance. It is your call on 3km Pokémon. For more information, see this buddy distance guide.
Community Day
Make sure you have enough gas in the tank, at least 50 or more miles left, if you think you might drive. Bring a bottle of water with you so you do not get dehydrated. Eat a meal before Community Day if it goes through the lunch hour.
You can search “Community Day” in a Facebook group or through Google in your area to see where others like to play, and the most frequently mentioned ones are usually the best spots. Driving is slightly better than walking around, but be careful to drive normally.
The rate for shiny Pokémon is 1 out of 25 during Community Day, when it is usually something like 1 out of 100. The three hours of Community Day every month is the best time to catch shinies.
Raids
Be careful about trespassing. Most property owners do not care if it is brief, but some might. Remember that you can avoid committing criminal trespass if you park on the street or stand on the sidewalk.
You get one raid pass every day. If you did not use yours up yesterday, you can do one with the previous day’s pass and pick up a new one to use today. They do not carry over any longer than that.
The best kinds of raids are legendaries, although certain 4-star raids can contain some rare Pokémon, such as Absol or Mawile. If you lose a raid, you can try again with the same raid pass, as long as the time limit has not expired.
Focus on Pokémon that do more damage rather than absorb more damage. You can run out of six Pokémon and rejoin with more, but you do not get more time to win a raid.
Raids often give you the best items in the game, such as Golden Razz Berries, TMs, and Rare Candies. Golden Razz Berries help catch legendaries, TMs change movesets, and Rare Candies are a faster way to get candies than walking a buddy Pokémon.
You cannot take on legendaries alone. People are usually willing to team up, so communication and coordination is really the key. I suggest at least eight people total, although the highest level players can go with fewer.
Join your local Facebook group and find a Discord link to your group. Make a Discord account and check out the area’s legendary raid chat. See if anyone has posted a raid in your area. If it is within fifteen minutes and you can get there in time, message them and let them know so they will wait for you.
Alternatively, a few people might create legendary raid events in advance on Facebook, and you can plan to go to one of them.
In terms of catching legendaries, large groups may split into Mystic, Valor, and Instinct. Someone will step up as leader; just listen to them as they will know how to optimize this. Mystic typically has the highest chance of getting a legendary because there are more Mystic players, and the team with the most players typically gets a Pokéball bonus.
Use lots of Golden Razz Berries for the catch. Curve balls are nice if you can land them, but I personally do not bother because legendaries are hard enough to catch as is. You may need to do a few raids before you get that legendary. Legendaries are time exclusives and will only last for a given number of weeks before Niantic rotates in a new legendary.
EX raid passes are available from raids at select locations. If you get an EX raid pass, you get a shot at Mewtwo. They will let you know days after a raid and give you a date and time to do the EX raid. It can be during a weekday, or it may be a weeknight or weekend. Make sure to arrive before the raid start time. Other players will probably not wait for late stragglers.