1. Split background to separate tiles (save as PNG)
2. Use as low as you can particles
3. Remove unnecessary objects from layout
4. more compress music and sounds
5. In event sheet, if you don't use certain variables, clear them (remove / free) (Like in c++ or pascal programming, when you create object and do not use any more - delete it).
Correct me if I'm wrong, but you really should read tutorials on scirra