To do this you're going to need to work with variables; I knocked up this little example (By using a platformer example from someone else that I happened to have open at the time, I'm very lazy and full credit for everything above my little comment on the event sheet to the original author!)
See here: sendspace.com/file/55neie
Basically I set a variable that tells the game what to set as the value for a Set Y Vector event, which gives you the bounce upwards effect (When you specify a negative value).
This variable is increased by with the function *1.2 every time the player hits the trampoline, so each jump will be a bit higher.
The next important thing is to reset the variable when the player touches the ground, this is easy to do with a set value event for our bounce variable.
And that's pretty much it, one event to handle your bounce height and your set: to make it better you can specify a ceiling(A numerical upper limit) for your bounce height by giving the trampoline event the condition that your bounce variable must be less than a certain number to continue growing in size.
Hope I've helped, any questions please ask! :)