Trainers are vital to any competitive deck, but many people still insist on
using them incorrectly or even worse, not at all.  Trainers allow you to
speed up your deck and be more effective by reducing the randomness that the
card game creates.
    There are really two types of trainers-hand manipulation and disruption. 
Hard manipulation allows you to control what cards you get, while disruption
allows you to control the playfield. 
    There are three types of hard manipulation trainers, drawing, searching,
and retrieval.  Drawing cards include Gambler, Professor Oak, Bill,and
occasionally Challenge.  These cards allow you to add more cards to your
hand, hopefully adding the cards you need.  Oaks and Bills should be a
mainstay in every deck (except stallers), but should be used with care so you
don't deck yourself.  Search cards include Computer Search, The Boss's Way,
Challenge (again, at times), Pokedex, Poke Ball, Energy Search, and so on. 
The only commonly used search is Computer Search, with Challenge occasionally
in particular decks.  These cards let you get EXACTLY the card you need at
the time you need it.  Retrieval cards include Itemfinder, Nightly Garbage
Run, Energy Retrieval, and Revive.  These cards let you return cards from
your discard to your hand (or bench).  These are particularly good when you
have a limited number of a certain card, for instance a fire deck may run
four Energy Retrievals.  These should not be used as search, but rather to
add to the number of limited cards.
    All other trainers fall into the category of  Disruption.  These cards
allow you to control the playfield by switching pokemon around, healing
damage or ailments, removing energy, adding damage, or otherwise changing the
playfield significantly.   Stall decks use disruption such as Scoop Up,
Pokemon Center, and Imposter Oak, while quick decks use Gust of Wind, Switch,
Scoop Up, Energy Removal and S.E.R., Pluspower as well as occasionally
Defender, Goop Gas Attack, Full Heal, Super Potion, etc.  These cards should
be used to counter your opponents strategies to prevent them from getting
their pokemon loaded, ready, and attacking.  These cards are used to get
quick kills, set opponents back a turn (and set you forward one), or deny
them important cards or kills.  Energy removal is a given, and the other
cards should be fit in to your deck style, playing style, and Metagame.
    Trainers should always be tailored to fit your deck (there's no point in
having a pluspower in a Damage Swap for instance), but should make up
approximately 1/3 of your deck (unless you play a Hayfever deck).  You should
never play singles (duh!) since this basically makes the card useless. 
Playtest your deck a little, if you find a trainer is basically useless, then
drop it and put in another that will be more useful.  Eventually, you'll find
the right trainers for you and your deck.
                                            -Foss, AKA FossilMagikarp
                                    Email: Edestus360@aol.com