What a great question. I am currently trying to figure this one out for myself as well. I'm in a long relationship and recently I've been trying to figure out how to spice things up because it has not been as passionate as it use to be.
Well first we decided to try new positions and experiment with anal. We also bought a vibrator and have been having some fun with that. However it did not quite solve everything. So my BEST ADVICE and what seems to be working better for us is to just communicate more with your partner. Tell them how you've been feeling and reminisce on the past to maybe rekindle that spark that burned so brightly and gave you all of that passion. Honestly for me, just letting my partner know how I feel and hearing how she felt allowed us to work on improving our sexual relationship. Right now as we are in the middle of improving it I can say I couldn't be any happier and am very excited to be with her and experience all the wonders of sex.