What Jed said. I always use a click track and a reference vocal/rhythm track which can always be recorded over when you lay down your keeper vocal/rhythm track. I also always do a count in at the start of each song. One more thing - if you leave too much time between laying down your instrumental tracks you start to hear things you "coulda/shoulda/woulda" done better. E.g. you learn a nice new lick that you want to put in. This is similar to the phenomenon that Rick described in his last paragraph above, except that in this case you don't hear mistakes, you hear things that aren't there and want to put them in. Then you'll never get finished.
All the best.