Whato all, a very interesting thread - I'm clearly fortunate as it never seemed to be an issue for me to play and sing at the same time. Guess I'm lucky, except..... on a few songs, mostly when I'm playing more complex bass lines, or playing a guitar style that is more tricky (Ragtime).
As
@Boydie said practise with a metronome by all means, especially scales, that'll sharpen things up (in the UK, guitar grades (exams) include playing scales, and these must be done at a certain bpm)
With regards to keeping in time, and there are a few scenarios here which might be of interest.
1. As a virtuoso guitarist/Vocalist - It should be natural to follow oneself playing, it's sooo flexible, that one can speed up or slow down when you want (rubato) one uses the gtr as your metronome, by the time your'e playing live you'll have nailed it anyway, I always say that learning a song and getting it ready to perform (or record in a paid studio) are a country mile apart.
2. As part of a band, you won't last long if you're not together, and the many hours practise one does alone or with the band will sort that out, seen a few very short auditions...
3. As a drummer or bass player - You
are the band's metronome, some pro drummers have a click track (or are made to use one) in their ear when recording. I find it
really, really hard to hit the beat every time, my drum kit has a natty trainer for this and it lets you know by how much each drum hit was off (late & early) and it's sometimes a bit disheartening seeing the results, makes one look like a complete buffoon, but the scale is in milliseconds
...
Respect to all drummers that can hold the beat, (I've played with quite a few that can't btw
, and done a few gigs as a drummer, one feels the sense of responsibility. Bass is a bit easier, but one gets the recursive issue of the bass following the out of time drummer and the drummer trying to regain the beat by following the bassist AAAAAARRRRGGGHH.
4. As a lead guitarist - this can be a little tricky live, just warm up well and don't wander off too far from what you know is safe, don't start syncopating 3 against 4 unless you can really do it.
I once knew a blind jazz keys player who had a wicked sense of humour and a keen pair of ears, the rest of the band would do their solos, and when it came to his turn, he would play the main themes from the other band members solos, and repeat their mistakes, very clever, just shows how exposing mistakes are.
Hope this helps
Rich