I've talked about this before, but I was just emailing with Ryan G about it and I thought I've gotta bring it up again: play through your mistakes.
Our natural tendency as players is when we fluff it, to stop, go back and play the part again, or stop, or start again from scratch...sure, during the early days of learning a new song you need to practise little bits over and over to force it into your brain, but once it is in your brain, my rule is if you are going to play a song, play it from start to finish and play through any mistake.
Most guitarists will start as half-song playing guitarists and that's fine - but arguably we should move on from there. If you are playing just for yourself and the dog, or playing for a packed venue, I believe you need to play a song from start to finish. To do that, heh, you need to practise playing a song from start to finish :)
Once I realised that it doesn't matter how many times I practise a song I won't ever be able to play it perfectly every time, I had to focus on how to recover after making a mistake. If your first instinct when you make a mistake is to stop, then you won't ever learn how to play through your mistakes. Sounds pretty straight forward doesn't it.
Keeping on beat is everything. If you have made a mistake but haven't dropped a beat, you know, it's probably only you who is going to notice. If that's the first thing you achieve from practising playing through your mistakes, then that is half the battle won.
Linking back in - you've made a mistake somewhere, now you want to get back on track. What chord are you in? What is the next chord? If you just fall back to playing some notes or strumming in the chord, and link back in on the next chord change, you'll be fine. This requires you to actually _understand_ the music you are playing - not just a sequence of notes. I fall down a lot in this area, being a programmable guitar playing robot. But by paying a little bit more attention to the music - and practising playing through my mistakes - I've become more able to fumble my way through most of my repetoire.
I could keep going...and often do...but I'll cut this one short. As my regular readers will know (there are 14 now Roman!) I have a set of JAW's Laws that not everyone agrees with; playing through your mistakes forms part of it. Tracing the lineage of this law it goes something like this: Play the guitar for audiences - audiences want to hear full songs - practise playing full songs - you will make mistakes - practise playing through your mistakes.