Open communication, open minds, and a mutual interest in your partner's pleasure as well as your own.
If you're thinking about specific things, I'll throw out some suggestions.
-Fantasies: Share sexual fantasies with your partner. Even if you don't plan on acting them out (and sometimes it's good NOT to act them out, depending on the fantasy), sharing such intimate thoughts with someone can be a major turn on. Have an open mind, and be with a partner who has an open mind.
Lingerie or dressing up: Sometimes simply wearing something different can add a lot to the bedroom. Wear it with confidence--your partner is not likely noticing every single flaw that you think you see, but is excited by the sight of you all sexed up just for them; it's a major ego-boost and a huge turn on to know that your partner is dressed up for you, prepared herself for you, and had you in mind while she was selecting the outfit.
Toys: Don't waste your time on someone who is intimidated by adding inanimate objects into the bedroom. Toys are meant to be used as more than just masturbators in most cases, and adding new sensations and variety is a lot of fun, and a mutually enjoyable experience.
Have sex somewhere new: Even if it's just the couch instead of the bed, or in a different room, new scenery and things to have sex on can add a lot of excitement.
Massages: Everyone likes massages, and it's a great way to extend foreplay. Putting your hands all over your partner's body, especially paying attention to areas that typically get less attention during sex, can really intensify things. Utilize your whole body and your partner's entire body during sex--a lot of things feel good and enhance other areas that feel good, even in the most unexpected zones of the body.
Try different things! Nothing ventured, nothing gained, right? Not sure if you'd like to be tied up? Have your partner tie you up! Not sure about spanking? Give it a try! Not sure about talking dirty? Talk dirty! Really, there are so many things you could do, all you have to do is use your imagination. I think a lot of people are hesitant about trying what they want, because they are worried about how their partner will perceive them. If your partner is judgmental, then you are wasting your time. You need to be with someone who is willing to explore with you and who is sexually compatible with you; otherwise, you're setting yourself up for boring and frustrating sex, which leads to resentment, which leads to unhealthy relationships.