Hard to stay motivated

I've gotten into a cycle of trying to lose weight for about 2 weeks (reasonable exercise, 1500 cal/day, yadda) and then slowly backsliding over the course of the next month. Repeat. I think it's the slow, slow backsliding that's killing me. Lose a pound in 2 weeks, gain it back over the next 4 to 6. And at this point, I can't tell if it's slight over-eating on my part, or if I'm not putting things in fitday, or if I've lost enough muscle tissue that I'm actually burning that many fewer calories.

I'm not really sure what I need to do to kickstart things. Or rather, I've got the kickstart thing down. Now I need something to keep me going, keep the veggies at my desk, keep the fires burning for a solid 2 months to ease me into bikini season. A pound every other week, that's all I want to lose, from now until the end of June.