Save, Save, SaveIf you're going to be successful at Pokemon GO, and if you're ever going to stand a chance at finding all 251 Pokemon currently coded in the game, you must learn fast to save your Stardust, Pokemon Candy, and Items. Don't waste valuable Stardust and Pokemon Candy on low-leveled, unevolved Pokemon - unless, of course, to evolve said Pokemon to increase your Pokedex or gain XP - but wait until you have a fully evolved Pokemon (Final Stage) before using them. This will allow you to save as much of them as possible, as well as giving you the most to work with when leveling up said Final Evolution into a powerhouse.
And the same goes true for your items. You only get a specific number of slots in your bag - unless you make the decision to purchase more space - so try to save and fill that bag as much as possible. You'll never know when a bunch of Potions or Revives will come in handy at a certain gym, or when you'll need tens of PokeBalls of all sorts to catch a rare, difficult-to-catch Pokemon.