Playing In Time = Tools for Listening
Let's use an example of a common error when performing a memorized piece of music to a backing track or live band. Let's say that you accidentally rest for 5 beats after a phrase instead of the required 6. You've worked on your song so much to memorize it that you have the spacing worked out between each lick so well that if you're not saving some of your mental power for listening to the band as you play to make sure that you are where you're supposed to be in the form, then you'll most likely stay out of time for a loooong time.
Here's a tool you can use...
Chord change is commonly telegraphed by the other musicians. In other words, each instrument commonly plays a series of notes to lead into chord change. The drummer may play a triplet on beat 4 to lead into a chord change on the following measure. The bass player may use the measure before the chord change to play a walking line (ascending or descending) that's commonly chromatic in nature, making it stand out from the more diatonic patterns they're playing up to that point. Sometimes this "pickup" is very subtle. It may be as simple as putting more weight (louder volume or slightly longer duration) into their notes before chord change. Even if all of the instruments don't play this pickup, usually one instrument does.... so if you're listening, you'll most likely pick up on the movement to a chord change.
The BIG pickup, and most obvious one, even for beginning musicians, is in the last two bars of the twelve bar blues progression called the Turnaround. If you get out of time, count on the turnaround helping you to get back on.
The other areas where you might encounter strong pickups are measure 4 (I chord) leading to measure 5 (IV chord) and measure 8 (I chord) leading to measure 9 (V chord). Simply stated, every four measures you'll commonly hear a pickup leading to the next four measures, with the BIG pickup being on third set of four measures (bars 11 and 12 of the 12 bar blues).
If you're working on a song with sheet music provided (like what I provide for you at BluesHarmonica.com), then you simply circle the downbeat note of measures 1, 5 and 9 of each chorus. Listen (without playing) to the original recording of the song and focus on hearing how the band leads into those downbeat notes in measures 1, 5 and 9. Sometimes a pickup will be strong and easy for you to hear and other times the musicians won't play any pickup... you can't count on it, but it's one of the tools we use to play in time. Now play your song, not focusing on what you're playing, but hearing the band and making sure your playing aligns with the chord changes you've highlighted.
We're all responsible for internal timing, but it's a fact that good musicians keep their ears open to the other musicians for influence. We tracking what the other musicians are doing to make sure we're playing in the right part of the form, as well as allowing what the other musicians are doing to influence our performance (dynamics, tone, body movement, etc.). If you're improvising, then the later is extremely important... what they play influences you, and what you play influences them.