Catch a Seasonal Pokemon at Any Time
Some Pokemon and items in "Pokemon Black" only appear during a specific season. "Black" determines what season it is in-game by what season it is in real life. This can obviously be incredibly frustrating and generally means you will have to wait weeks or even months to catch a certain creature you need to complete your Pokedex. However, this can be side-stepped easily. You can change the in-game date by changing the date set on your DS, which can be accessed through the console's main menu. By doing this, you can set your game to any season, making all seasonal Pokemon and items available to you.
Gain Experience Easily
Capture the Pokemon you want to level, and then put it in your party. Go to Route 3, visit the Day Care Center, and tell the staff you want them to take your Pokemon and train it. Pokemon train at Centers according to how many steps you take in the game world. Now travel to Nimbasa City and go to the Battle Subway area. There will be a path which circles a column in the Battle Subway, which you should walk onto and stand in the middle of. Hold down the "Down" and "A" buttons to start walking. As long as you are holding these buttons, you will continue to take steps, and the more steps you take, the more your Pokemon will gain experience and levels.
Easy Pokemon Capture
In older Pokemon games, there was a method for making Pokemon easy to capture by pressing the "A" button repeatedly after a Pokeball has been thrown and the Pokemon is trying to escape. Though never officially confirmed by the creators, this method is widely considered to help quite a bit. There is a similar method for "Pokemon Black." When you throw the Pokeball, and the screen zooms in on the Pokemon trying to escape. Use your DS stylus and draw circles in a counter-clockwise direction around the Pokeball. The more steady the pace and the more accurate your circle, the better your chance of capturing the Pokemon. This does not guarantee a capture, but it is known to help.
Action Replay Device
As with all other Pokemon games, the only true "cheats" -- those that aren't just exploitations of the game's programming -- are those activated with an Action Replay cheating device. Action Replay devices can be purchased from Code Junkies, and are inserted into the Nintendo DS where the game cartridge would normally go. Action Replay cartridges come with some codes already installed on them, and it may recognize your "Pokemon Black" game automatically. If it does not recognize the game, the Action Replay will give you a code for the game, which you can put into the device and save as "Pokemon Black." Cheat codes can be found at various websites such as supercheats.com or codejunkies.com.