There are so many ways to spice up your sex life because everyone isn't the same, but there is one thing that everyone has in common. Everyone has someone that they are attracted to, whether it be the mail man, the plumber, actor - whatever. The best way to spice up your sex life is to imagine that you are having sex with the person that you find so attractive. If you really want to be turned on, then don't tell your partner about it, let it be your little secret. We as humans seem to be more excited when we are doing something we know that we shouldn't be doing.
Another way to spice up your sex like would be to role play, or create an altar ego that would do and say things that you know you would never ever say or do, and I guarantee that you will have a great time every time.
I know that most people won't agree, but when you are single you like to test the waters. Many people would be against having multiple sex partners, but it does happen so I will say before I get into my next suggestion is to always be safe and wear a condom. If you are single and you want to test the waters and see what's really out there, then you should go out and about; don't commit to anything or anyone just go out and have fun sex anything and anyone with no care. Not only will you learn about yourself, but you will also feel freedom that you don't feel when you are in a relationship, not to mention you will be stress-free.
Now for all of the couples that don't mind adding someone in on their fun. You can always get someone to come in and feel you place. You can have sex with them all at once, or you can watch as your partner has sex with them. Get in with other couples who want to have the fun and switch partners for the night; I promise you will have so much fun.
Like I said before, everyone is different and enjoys many different things. Do what you enjoy to spice up you sex like. Make a movie, invite friends, go out and freak the town - whatever floats your boat. Just be safe and have fun. Remember; it's your life, you live it.