The two best ways I've found are by Martin Berkhan's method on Intermittent fasting diet for fat loss, muscle gain and health or Lyle McDonald's Ultimate Diet 2.0 on www.bodyrecompisition.com.
I am doing the UD2 right now and it is much more effective than Martin's method. It's also much more demanding and much more extreme. If you're looking to cut the last bit of body fat in the next 6-12 weeks while maintaining or even building muscle, look into the UD2. If you want to see the results by summer 2012, Martin's methods should get you there.
It's very possible to put on muscle and lose fat at the same time. You have to essentially go through anabolic and catabolic phases. However, I've only really found it possible on a low fat diet. You would have to eat low fat, low carb and very low calorie the first half of your week and burn through your glycogen stores with high repetition, high intensity full body exercises. After that, you spend the second half of your week eating large quantities of carbohydrate and protein, but still low fat while lifting heavy weights for low repetitions full-body style. Essentially, you take in lots of calories and protein, the carbohydrates upregulate your metabolism, boost insulin and leptin, but go to glycogen instead of fat storage while the protein goes into muscle building and repair. The only thing to get stored as fat in the process is, well, dietary fat, so you have to cut that low. You also need to know what your max carbohydrate load is so you don't overfill your glycogen and spill over into fat. I eat around 1,200 - 1,300g of carbohydrate from Fri-Sun and probably 500g of protein with maybe 150-200g of fat total. It's not exactly fun, but it's working better than any method I've tried so far.
Don't put your trust in anyone on this forum, including me. You are the key to your own success.