Aimtrainers are alright. But when it comes down to it just spending time, in the game you are trying to get better at, is going to be the best option. Just learning how to move a crosshair onto an object faster or more precisely will only get you so far. A lot of games, especially FPS titles, have unique models, textures, and environments. Take the time to learn what you need to target and just practice at getting a faster time to target reaction.