1.Buy cards online, rather than fishing in Booster Packs.
They call them, "Ultra Rare" for a reason.
2. Buy plenty of Draw Supporters and "search" cards. "Ultra Ball, Professor Juniper, N, Bicycle, Bianca, Cheren, Skyla, Random Receiver (Colress and Ghetsis provide less consistent draw, but potentially powerful effects). Level Ball, Heavy Ball (the last 2 are situational, one can only find Pokémon with 90 HP or less, and the other can only find Pokémon with 3 or more Retreat Cost.)
The typical amount of "Draw" cards in a Deck is 12-14
3. You'll need multiple copies of the same card, even Pokémon, in order to gain the upperhand. You can have a maximum of 4 cards of the same name in a Deck (60 cards). Stadiums are becoming more popular so get some Virbank Gym, Skyarrow Bridge, Pokémon Center, and Twist Mountain.
4. Staple Trainer Cards: Pokémon Catcher, Hypnotoxic Laser, Switch, Enhanced Hammer (to counter Team Plasma Decks,) Virbank Gym (this and Laser are very related).
5. All strong Decks run Pokémon-EX, such as: Mewtwo EX, Darkrai EX, Rayquaza EX, Keldeo EX, Landorus EX, and Cobalion EX. The first four have a Promo version, and are therefore much cheaper.
Sigilyph from Dragons Exalted provides a hard-counter against Pokémon EX, and is much cheaper than they are. Get it.
6. Strong Decks run much lower Energy than you think: 10-14.
7. Check out the "BW-On Metagame Discussion Forum". It has example Decklists.
8. Practice on Bebesearch.com and playtcg.me : The first website allows you to construct virtual representations of a Pokémon Deck, completely for free. The second website is where you can battle other players online.
You may need to press CTRL - (minus) once or twice when you enter a battle. There's a tab where you do all your actions (Draw Cards, Set Prizes, etc).