One of the big problems that I notice when students are practicing is that they want it now, now, NOW! They want to be able to make that chord change, to play that song, to be able to play at xBPM etc. Whilst it's great to have goals they often try to arrive at the goal too soon without putting the correct work in. Consequently as teacher I find that I need to backtrack with the student to cover their learning again and in some cases build things back up from scratch. It's not an easy thing to do as more times than not the player is wanting to play what they want to play as the finished result without taking the time to get it right!
Take the example of a chord change in a song. Let's say that you are strumming the song in time and you know that you need to change to the next chord in 4...3...2..1...... ....... then there's the gap! The time it takes for your fretting hand fingers to move from one shape to another. This may only seem like a moment but it may throw you out of sync with the beat. More importantly if you are playing with others you may loose the musical synchronicity (i.e. go out of time with them) and the music may get a bit messy! It's worth noting that this applies to anything that you are trying to learn or apply to your playing, be it a song, lead lick, rhythm figure or even when practicing improvisation.
Some people may simply just try and play it again and again without taking the time to focus on what's really causing the problem. Practicing the whole song won't really help you much with this. However practicing the chord change on it's own will help more as it will be a better use of time.
You can use a metronome for this although anything with a regular beat will do. Start at a sensible pace such as 60BPM and work with the beat in 4/4 time, so counting 4 beats along with it. On the first beat play the first chord and wait for beats 2, 3 and 4 before you change to the next chord on the next time beat one comes around.
1 2 3 4 1 2 3 4 1........
X X X........
In the time between the beats, i.e. on beats 2, 3 and 4, you can be changing to the next chord. Make sure that you do this straight after finishing the first chord to give yourself the most amount of time to make the change so that you are not rushed and practice this repeatedly until you can make the change comfortably. Notice I didn't type QUICKLY. You need to just focus on comfort and easy. The speed will come in time.
Next play along withe best again but strum on beats 1 AND 2, leaving beats 3 and 4 to make the change to the next chord. This gives you slightly less time to make the change but will give you a little push to make it faster. Being that you are already use to working with the beat in the first part of the exercise. This forces you to make the change quicker but not by a lot. Therefore you will increase your playing speed within a controled confort zone.
Then, play on beats 1, 2 and 3, leaving 4 to the change. Finally when you are finding this comfortable, play on beats 1, 2, 3 and 4 leaving only the gap between beats 4 and 1 to change. By this time though you will have taken the time to play the change with a gradual reduction in the amout of time needed to make the change, making sure that you are comfortable each step of the way before increasing the difficulty of the exercise slightly.
Bear in mind that this may take several days to get this to the end, or even several weeks if you are a beginner with not much time to practice. However the process outlined ensures that you are comfortable with playing something before you are moved forward to the next step. This also work very well for learning a whole bunch of other skills, be them musical or not.