(If you want to listen to advice from the inexperienced)
It seems to me that sex is just physical stimulation of nerve endings. Basic sex will only get you so far, and if you are using the same partner week after week than you will at some stage think "been there, done that".
When it comes to using working ladies, everything is pretty clear. Here's the money, here's the fun, see you later.
But when it's your partner (especially a long-term partner) then you need to bring something else to the table. You need to (at the risk of sounding like a snag or something) connect on a deeper, emotional level. You need ... love.
Okay, now that I've lost everyone's attention - there are many ways that you could try to 'bring the spark back'. And most of it will involve something unique or special to you and yours. So what works really well for someone else may not work at all for you.
Having said that, here is a short list of very general ideas that may help you, or give you other ideas. Feel free to laugh and mock me...
Date nights: (cue everyone rolling their eyes) I know most people will laugh at this suggestion, but think about it for two seconds. Remember when you first started seeing each other? Did you just snap your fingers and they jumped on your lap? Maybe, but probably not. So if you take the effort of making a special night, where the focus is on the couple then that will make it special. Formally ask him / her out, get flowers, go somewhere special - basically, pretend you are trying to whoo them, rather than just spending a night with your partner.
Get into shape: Everyone can be fitter, unless you are a gym instructor. Life happens, so it's easy to start 'letting yourself go'. You don't need to build muscles, you want to concentrate on building stamina. Even just going for a walk will help. Even better, take your partner with you.
Do something for them: If you are sitting back thinking "you don't turn me on lately", then how might they be thinking? How is this for an idea: have an evening where your entire focus is on making them happy. Forget about getting yourself excited, spend all your effort on making them excited and satisfied. I'm not just talking about sexually, but emotionally as well. And hopefully they'll want to return the favour.
Talk: (cue more finger-pointing and laughing) It sounds a bit flippant, but when was the last time you sat down and just talked with your significant other? And not just about the housework or your jobs or who's turn it is to empty the kitty litter box. But just talked about what you like and don't like, your hopes + dreams and all that kind of stuff?
As I said at the top, I'm not the most experienced chap here, but I do know a bit of theory. I don't know if any of this will help you, but it may be worth thinking about, and seeing how you could apply something to your life.
Or if you come up with something else, feel free to share...