Much could be said in this regard, but let me mention a couple of simple things that you can do immediately to improve your improvisation.
1. Practice playing continuous 8th notes throughout
I got this tip from a guitarist who in turn got it from Jim Hall during a private guitar lesson.
Practice improvising throughout the tune using only 8th notes (i.e. two notes on every beat) and without any rest.
This will force you to become fluent with the whole piece, because you cannot take a break from playing when you run out of ideas, and you cannot mechanically fall into some habitual lick.
Once you're able to play continuous 8th notes throughout, go back to improvise in any way you like, and you'll find that you can do it a lot better than before.
2. Think rhythm first, notes later
Decide in advance the rhythmic structure of the next phrase, and once that's decided, improvise the notes that will fill it.
You can also write in advance a few rhythmic structures on a piece of paper, and then, while you improvise, follow one or the other.
After you do this for a while, and become good at filling up with notes some pre-decided rhythmic patterns, go back to improvise freely, and again you'll find that your improvisation quality has improved.
(The latter tip was mentioned by Dizzy Gillespie as one of his own strategies during practicing and live improvisation)