This works in the same way as the previous games. Normally you can only save one car in your garage, but with this you can save two or more! All you have to do is get your two or however many cars and bring them up to your garage. Drive one of them until the garage door opens, and then drive it half way in and get out of the car. This will keep the door open for you. Next, get into the other car and drive that all the way into the garage. Then get into the car half way in and drive it the rest of the way in and get out and leave the garage. Et viola! You now have two cars stored in your garage!
First browse to your GTA:SA isntall directory, it will look something like this:
C:Program FilesRockstar GamesGTA San Andreas
Then go to the data directory. MAKE A BACKUP OF ANY FILES BEFORE YOU EDIT THEM. To edit the vehicle stats, open the handling.cfg file in notepad. As you can see, all the info on every vehicle in in there. Follow the instructions in the comments to find out how to edit it.
You can do anyuthing like make a Rhino as light as a PCJ600 and control just as well.
Basically, you can edit anything in the game from these files. Just be careful what you edit and leave them as sane values to prevent glitches.
During Woozie's mission "Amphibian Assault", the wanted level system is disabled. You can now travel everywhere in the game including the restricted areas like the Eastern naval base and all police stations without the cops chasing you. This is time to collect the two oysters in the extremely dangerous naval base without being shot.
For free and fast car repair, park your car in one of your garages, back away to let the door close, then step torwards the garage so the door opens again. The car will be instantly repaired of all dents, etc.
This is especially useful if you have an expensive modded car that you want to keep for certain ingame stats.
During the mission "Reuniting the Families", you have to shoot down a group of SWAT team and then enter the motel to save Sweet. Beating the SWAT, instead of getting into the motel, you can travel all around the entire map and enter all the restricted areas like the military base and the maps that are locked. You will have no wanted level and no cops will chase you. You can explore the entire map freely without taking risk. This is the best time to confirm vehicle and item locations and to collect the snapshots, oysters and horseshoes.