Which is the best in game configuration that gives the best looking in game aspect and the most FPS?! And what "tricks" can be made to improve the FPS without giving up at the quality of the game?.
I've found with most games, if you want to keep the nice-looking effects but increase your FPS, that running at a lower resolution tends to help. Everything will be slightly bigger on the screen, but you'll be able to leave your video settings up higher and still get a decent FPS.
If you do want to keep a higher resolution, you'll have to lower your video effects. In general, effects that are active much of the time (like anti-aliasing, vsync, particle effects, or high quality shadows) tend to be the ones you'll want to turn down. Some things like 'Bloom' are usually alright to leave on though, because there isn't a whole lot of constant processing involved.
Note: Some games don't like being run at anything other than the native resolution, especially on LCD displays. Most games won't give you any trouble, but if things start looking strange, you'll have to modify your video effects instead and leave the game at full resolution.
It really depends on how you determine 'quality' though. A high resolution with lower graphics settings and effects can probably get you the same framerate as a lower resolution with higher effects. However, in my experience, lowering the resolution seems to make the most noticeable difference to the framerate.
If you play on a CRT monitor, running the game at a lower resolution might be the most logical solution since a CRT monitor can display different resolution without the help of interpolation.
However if you're playing on a LCD panel display, you'll want to stay away from this option as LCD displays will rely on interpolation to scale down the image and try to display it at the native resolution of the display. This results in loss of image quality.
Personally, I can't stand running a game at anything but the native resolution of my display. So in order to increase FPS without hurting the quality of the game, I would start by lowering shadow quality, motion blur and filtering options (AA, FXAA and others). If that doesn't cut it, sadly you'll have to start hacking away at the core quality settings of your game...
Like suggested in the comments, another option is to lower your game resolution while keeping the game at 1:1 pixel ratio. This will eliminate the need for interpolation, resulting in a pleasant experience with likely higher in-game graphical settings. Most graphics card option panel will let you force this display behavior.
I'd say the biggest FPS hogs are Shadows, FXAA, AA, Anisotropic Filtering and view distance/load distance (if applicable). Lower end GPUs with high VRAM can handle high textures and pretty decent resolutions, but you're gonna need higher end GPUs to handle all of the little FPS hogs that I mentioned. "Tricks" for improving FPS are extremely dependent on the game/game engine...can you give an example of a game?
Edit: Tag was changed to League of Legends. For this game, you're going to want your native resolution on your monitor. Turn down/off shadows and AA and you'll notice a massive increase in FPS.