One of the best guitar tips I ever received was to practice with a metronome.
Sure, it’s common knowledge that a metronome can help you learn a piece by letting you work your way up to full speed. The special trick I learned, though, was different.
One day I had to learn a part for guitar in really short time. Normally I could learn a pretty simple part in a night or two, or a more complex part in about a week. This was a complicated part, and I only had two days. My friend suggested that I learn it as usual, starting by playing at about half speed and working up to full speed as I got more comfortable.
The real trick to it, though, was not to stop a full speed. Instead, I was supposed to go up another 25%. By playing the part faster than it was supposed to be for a while, going back to regular speed felt like slow motion. Even when I wasn’t completely comfortable, I felt like I had enough time to make an adjustment.
Ever since then I have used this trick to learn guitar parts faster.