Choose invitations that actually look inviting. Thick, high quality paper with bright colors or a unique design that makes the envelope stand out from other mail that may arrive on the same day. Your invitations should excite interest even before they are opened, and if you have chosen your design well, they should evoke reminders of the happy couple as well!
Perhaps there is a hobby that both of you love to do. You can use this 'theme' in your wedding invitations as well. If you both love to fish, you could use fishing line and lures for your invitations, or a hobby of wood_working could provide you with a way to expand your hobby into creating some special invitations. Regardless, if there is a hobby that the two of you share that helps to define you as a couple, don't be afraid to use it to your advantage and choose the best part of your hobby to direct your unique wedding invitation choices.