Without doubt, the best thing you can do is practice, even to a point where it gets boring. Okay, you may have heard a set over and over, but your audience won't have heard it, and if by your hard work has produced a top-quality set, believe me, they will appreciate it!
One of the guys also mention the crossfader. When I started mixing with vinyl, there was no such thing - each deck had it's own volume slider, and in my opinion, you still get better mixes that way. I have tried using a crossfader, but the mixes don't sound so good to me.
For music choices while you are practicing, I would choose a style of music and stick to that for a while. When I got back into mixing I used house music, as it has a fairly consistent bpm, usually around the 125bpm mark. I spent hours trying different combinations before I seemed to develop a sense of which track went together well.
One good thing to remember is that even though two tracks might have identical bpm's - it doesn't mean that they are going to mix well. There is also the issue of which key the song is in - the closer two songs are in key, the better the mix will sound. This is called 'Melodic Mixing' and it is worth looking into.
Just keep up the effort - even though you think you're not improving, one day you'll do a set and think 'Wow! That was really good!'
Best of luck!