A thread to post all the great tricks you know to use in JCS in.
I don't know many myself, but if many experts decide to post, this could turn into a great collection of useful tips!

I'll try to think of some that I know myself..

Custom enemies
You can create custom enemies by using animations and hurt sceneries, however, this only works in a tileset that supports something like that. By making your own tileset, you have many new possibilities using this idea.
For example, a walking enemy walking three tiles could be made by using animations as follows:
( 0 = empty tile, 1 = enemy animation )
10000
01000
00100
00010
00001
00010
00100... etc.

This can also be used for guns shooting bullets, things like meteors, moving spikes, etc.

SFX
You can create a temporary rumbling sound by placing several rocks behind 3rd layer tiles in a closed space and placing an activate rock scenery close to them. The rocks will go crashing against each other for a while, then stop.