Large intervallic leaps and interesting rhythms can make phrases more compelling. The most effective place to use both devices is at the beginnings and endings of phrases. Intensifying one's focus on how one starts and concludes improvised musical phrases has added benefits as well.
Some ways I like to practice this vary from very general to quite specific. Here are some examples:
Start every phrase with a large leap (P5 or larger).
Start every phrase with a large leap, continue with a line in the opposite direction.
Start every phrase with a large leap from one chord tone to another.
Start every phrase with a large leap from a chord tone to a tension, resolving to another chord tone.
Start every phrase with a leap of a [3rd, 4th, 5th, 6th, 7th, Octave]
Start every phrase with a leap of [m3, M3, P4, A4, P5, m6, M6, m7 M7]
Start every phrase with an ascending leap.
Start every phrase with a descending leap.
Alternate starting phrases with ascending and descending leaps.
All of the above can be applied to ending phrases as well. I find it helpful to focus on one or the other at first, then investigate ways of combining them:
Start every phrase with an ascending leap, end with a descending leap.
Start and end every phrase with a leap of a P5 (or any specific interval).
Start every phrase with a P5 (or any specific interval), end with a M7 (or any different specific interval).
Etc., etc. . . . there are practically infinite variations you could some up with.
You can do something similar by picking a specific rhythm (usually involving a syncopation) or a few rhythms to work on starting and ending phrases in rhythmically interesting ways. You can make this more challenging by displacing the rhythm in various ways (e.g., 8th-quarter-quarter, start on 1, 2, 3, or 4, or start on any of the offbeats).
Working on these ideas overlaps somewhat with the notion of soloing using rhythmic and melodic motives, and will make your improvisations more cohesive.
An additional idea: try to start each phrase with the same note that you ended the last phrase with. This is a subtle trait in a lot of music, once you start analyzing music with it in mind, it's all over the place. I think it comes from the fact that when singing a new phrase, it's much easier if it starts on or near the last note you sang.
This idea can be used as a standalone exercise, or a way of building on another exercise (you could use it as an additional parameter in any of the above suggestions).