Once you have reasonable control of playing eighth notes anywhere within a bar, try combining eighth notes and rests on or off any beat, placing them where you think sounds best.
Another popular rhythm variation is a two bar rhythm. Try the following example which combines eighth notes and eighth note rests for the duration of two bars. Listen to the recording to help get the right feel of this one.
Be sure to memorize the timing of the set rhythm carefully before attempting to improvise with it.
This whole method of using a set rhythm may seem a little strange at first, but stick with it. The idea is to get control of where you want to play and where you don't want to play rather than just playing the same old licks out of habit. Most of your memorized licks won't work with a set rhythm, and this forces you into new ways of playing.
Make up some of your own, the possibilities are endless. The hardest thing to do here is to keep the set rhythm in your memory while you are trying to improvise with different pitches. The difficulty increases as the phrases get longer, but as your musical memory improves, so does your playing. After you have control of two bar rhythms, try four bar rhythms. You could also try odd numbered bars like three or five for unusual phrasing.