Wow - very interesting thread. I don't really believe in the existence of a 'Mojo' per se, but I think the comparisons people have made here are right on: work, relationships, exercise etc.
I've been in the salsa scene around two and a half years now, and I've gone through periods of being out 5 nights a week, to only making the one. But for me it's all about expecting too much of yourself and throwing yourself in at the deep end. I haven't had this with salsa, but when it comes to running regularly I have started and given up about 6 times in the last year. There are three things I always try to remember.
It's a marathon, not a sprint
Progress is gradual, not instant. And as others have already said, getting fed up with a perceived lack of progress is bad, and very demotivating. But as Jake mentioned, you're just progressing at a slightly slower rate. Throwing yourself in at the deep end, no matter how hard you try is not going to give you the results you want. It will stress you out, tire you out and make you fall out of love with the idea of doing it at all.
Making habits takes as much time as breaking them
Most people have an objective when they try something new, be it with dancing, exercice, work or whatever. Make sure your objectives are achieveable or you're setting yourself up to fail. If you want to get really nerdy, and like nonsense business methodologies check out the SMART model.
Once you know what you want to achieve, think about what you're going to do on a routine, figure out when you're going to do it, and then do it. And don't allow yourself any excuses. If you don't impose a structure on yourself then you're likely to slip.
That classic quote "pain is weakness leaving the body"
I'm not sure who said this first (and see no point in trying to verify) but I think about it a lot. In a very literal sense, with physical pain, remember you are putting yourself through it for a reason. To better yourself. And by the same token, if you stop and fall out of your routine, you're going to have to do it all over again and it will be twice as hard. In a slightly more metaphorical sense you can think about this with any challenge you have been set. Challenges are so-named for a reason - they're going to be tricky, and perhaps even "painful". Just remember why you're doing it in the first place and then the pain will be a distant memory!
Finally, bringing it back to Salsa, I mentioned I don't really have this problem - at least not as a dancer. I do as the DJ. It's a constant battle for me to try and keep things fresh, and finding the time around the rest of my schedule. But nothing makes me fall in love with it more than when I have something new, which is my take on Jakes #1 piece of advice above. Mixing it up tricks your mind into thinking that you're going on a whole new adventure, takes away from the monotony, and presents you with a whole new series of challenges and possibilities - and nothing is more motivating than that!