If a game is too big few people will take the time to download it. The internet access I get is constantly cut off by phone calls ect. so I only dload games below 1mb. ok... sometimes bigger but only if the game has REALLY good reviews.

Graphics:

If your game only uses 256 colours DO NOT set the colours to 16 million, this makes the game a lot bigger than it needs to be.
If your game uses photos...
1. Try to change them for hand drawn graphics
2. Make them smaller then stretch them in the game
3. Make them darker or lighter - this only lowers the size a bit but it's better than nothing.
If your game is located in space, consider using a mosaic backdrop for the stars.
If your game has a plain background, don't use a backdrop - change the level properties (if you don't know how, it's under edit-->level properties)
If your game is low resolution but has a big window size, shrink it to sompthing like 320x200 and tell it to 'resize to fill current window size' and be maximised on boot-up.
A good way to make a 320x200 game fill the screen without slowing down too much is to use the set resolution object, set the resolution to 640x480, then go to the game properties and set the size to 640x480, check the resize to fill window option but NOT the maximise option. Remember to 'DESTROY DD OBJECT' at the END OF GAME.

Code:
If you use alot of repeated code, take it out! you can use the fastloop object instead.
If all your levels are the same except the layout (same graphics and gameplay) only write the code for one level then tell it to load in the levels (for a basic example of this see my Mouse Maze game. for a better example see Gravitorb2).

Sound:
Try to use the same sounds over and over again (not too much or they get anoying), instead of inserting new ones, the more sounds you have, the bigger your game gets.

Movies:
If you include movies in your game, make them low res this saves alot of space.
When making the movie think about how fast it will play, slow movies need lots of frames to appear smooth, but with fast movies, you can skip frames.