Practice tips and tricks
If you're trying to learn a difficult chord change, passage or anything that does not come easily, firstly you must slow down!
Don't try and play at full speed when struggling. Instead, using a metronome practice playing correctly at say half speed. By doing this you're allowing yourself a chance to see how mistakes are happening. By practising at full speed from the start, you're just reinforcing your mistakes over and over.
If half speed is still to fast then go slower still. There is no shame in slowing right down to get you're head around what's going on. Get the strings doing what you want them to do before speeding up.
Break it down
Is what you're trying to learn longer that 5 or 10 seconds? In a fast solo a lot can happen in 10 seconds. Therefore break it into manageable chunks. Repeat the first 5 seconds over and over until its second nature. Then work on the next 5 seconds. If that's easy then break it into larger sections. What you don't want to be doing though is trying to memorise too much information in one go. You'll overload your brain and end up remembering very little or nothing at all.
Good days / bad days
Have you ever found you've struggled to do something that you'd normally consider second nature? Perhaps you have an 'off day' driving your car or you've had a bad day at work.
This happens when learning anything on the guitar. It can be very demoralising when you think you've finally cracked something to then find your messing it up the next day.
It's important to keep the faith though! The more you practice and write to memory the less bad days you'll have. If you do have a bad day it will be a lot better than your 'old' bad days.
How long should I practice for?
I have been told we as humans have a pretty short attention span. This is certainly true for me. After about 20 minutes of practice my concentration starts to waiver. Therefore I stop practicing, have a break and do another 20 minutes or so later on. Two or three hours non stop for most of us will be far more unproductive that two or three hours broken up into blocks of around 20-30 minutes.