Watch her masturbate
Knowing that you would really enjoy watching her bring herself to orgasm gives her a great deal of power, and she would, of course, love to please you. But, revealing herself like that in front of you is a big step for any gal. If you want her to do this, then sharing your fantasy with her is an excellent way of "suggesting" she actually do it.
A good method to start her off is to prompt her while you are actually having. It is easier if she is facing away from you and her needs stimulating -- take her hand and hold yours over it on her, and then she can set the pace and you follow suite.
After a while, take your hand away. She may or may not keep her hand there, but very gently insist it and be sure to show your extreme appreciation -- she won’t want to spoil your fun! This should develop into her feeling much more comfortable touching herself in front of you, and one day -- maybe -- she will give you the whole shebang. Don’t be forceful, and if she is showing signs and symptoms of being uneasy, don’t push it -- don’t even mention it again, because one day she might just decide to loosen up all by herself.
Watch her with someone else
This is an interesting one, and one she isn’t likely to understand, but neither will she feel threatened by it. It is something she can get her head around -- she is the one with someone else, not you -- and therefore she will probably be interested in exploring it further -- in your imagination, of course. This other person could be a man or a woman -- whatever takes your fancy -- and only involves complete and utter voyeurism.
This actually happening is unlikely unless you really, really, really want to do it, but even then she probably won’t ever agree. It is the sort of thing that may turn a man on rather than a woman, and for this simple fact it's probably not going to ring her bells to any significant degree. Unless you’re both into swinging or other "sharing" activities, watching your girlfriend getting screwed by someone else is highly unlikely to ever happen to you in your lifetime, unless you walk in on her by accident