Tips for pressing flowers using a microwave
The following might be suboptimal, but it worked several times for me (and is purely empirical):    


Set the microwave at minimum power.  
Put the flower in a folded paper towel which you place in a tupperware.   
Put the tupperware a few seconds in the microwave.   
Open the tupperware, change the paper towel and dry the vapor in the tupperware.  
Restart at step 3 until you are satisfied.  


Of course, the number of seconds depends a lot on the type of flower. Typically, I start with 5-10 seconds and then decrease the amount of time. I never did it for flowers with petals, but I guess that you'll have to reduce the time in this case. Note that keeping small times allows a better control on your experiment.
I should admit that I never could get it as dry as after a long natural process, but you can definitely boost it with this technique.