Adding Random item drops (RPG)

I've been having an issue coming up with a good method to implement random item drops when monsters are destroyed in an RPG i'm working on. I need to come up with a way to do the following.

1. set the odds that an item will drop (10% chance each time)

2. Some items I want to drop and then never be able to drop again

3. some items have increasing/decreasing odds of dropping.

Any suggestions on good ways to implement this?

You need a lot of variables for that, then just do On Enemy Death, sub-event: Random(0,100) less than or equal to Dropchance, Action: Drop Item X,Z,Y

Some items don't drop, just do a modifier at start of layout for a stage you dont want it to drop, make Dropchance to 0 and it will never proc on death.

wow. thats so simple I can't believe that stumped me. Looking at a problem to long leaves you blind i guess =)