One way to lose in the Pokémon TCG is to have no cards left to draw at the beginning of your turn.
Is there an combination of cards that prevents you from ever running out of cards to draw?
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Sign up to join this communityLysandre's Trump Card reads:
Each player shuffles all cards in his or her discard pile into his or her deck (except for Lysandre's Trump Card).
This can be used with any card that returns a Supporter card from the disard pile, such as VS Seeker which states:
Put a Supporter card from your discard pile into your hand.
There is no card that guarantees you won't deck out. Pokémon doesn't work that way generally, the card effects are simpler than Magic so you don't have the kinds of cards that alter very specific rules like that usually.
Most games won't risk decking out unless you're playing a particular kind of deck that causes you to draw a lot (Zoroark for example). Those decks typically include several cards such as Cynthia (UPR 119), N (BW 100), Judge (FL 108), Hala (GR 126 ), or similar, to allow the player to shuffle their hand back into their deck periodically to avoid large buildups of cards. Some of those allow you to force your opponent to do the same, which can be good or bad depending on your opponent's deck situation.
Other answers mention the existence of cards that allow you to take cards out of your discard pile and back into your hand/deck; I haven't found that to be common in the current meta, as usually the games are fast enough - particularly with the emergence of the GX meta which means you only need to defeat three pokémon to win - that you're not going to deck out unless it's from drawing in most cases.
Other than that, though, Pokémon deals with decking out by making it easy relatively speaking to draw particular cards out of your deck, rather than forcing you to draw large numbers of cards to hope for one. Ultra ball (SM 135), Nest Ball (SM 123), Timer Ball (SM 134), and many other 'search' cards allow you to typically set up your archetype quickly without needing many turns to get it going, not to mention the Tapu Lele / Brigitte combo that appears in nearly every major deck in the current meta; even my relatively slow Metagross deck usually is fully operational by turn 5, with maybe ten cards in the discard pile, and then takes maybe 5-8 more turns to win against nearly any opponent.
Edit (1/2020): One card that's been added to the rotation since this was originally posted is Lillie's Poke Doll, which can be used in an infinite loop of placing it down on the table, then immediately putting it in your deck (including a deck of 0). This will not prevent you from decking out if your opponent is actively milling you (as they can simply mill Lillie's Poke Doll), but it is used in some stall decks to extend the game (in addition to its ability to take zero prize card knockouts).
As I understand it, both of the cards Nova mentions will be rotating out of Standard Format on Sep 1, 2018 when the 2019 season format rotation takes place. There are several cards in Sun & Moon that will allow you to put some cards:
Gardevoir-GX: (GX attack) "Shuffle 10 cards from your discard pile into your deck. (You can’t use more than 1 GX attack in a game.)"
Rescue Stretcher: "Shuffle 3 Pokémon from your discard pile into your deck"
Energy Recycler: "Shuffle 5 basic Energy cards from your discard pile into your deck"
Some Pokemon (Kartana-GX, Butterfree) return themselves and attached cards to the deck: "You may shuffle this Pokémon and all cards attached to it into your deck."
Then, if you are holding a big hand, there are cards like Hala and Shaymin that would allow you to shuffle your hand back into your deck. These have specific draw requirements for refilling your hand.
In addition to Lysandre's Trump Card in Joe's answer, there are quite a few cards that prevent decking out indefinitely outside of the Standard and Expanded format.
... And a few others. Anything that puts cards back to your deck as a Pokémon Power or as a Trainer that can be grabbed back easily from discard pile.