Combine it (fast/slow) and don´t have it too long (solo). ca 45 sec. is enough. And don´t be afraid to hit a note and hold it for like 2-3 sec. Just make sure to do somthing with it (heavy vibrato, bend, use the whammy bar etc) and than pull of some of your fastest licks (they should be fast but easy for you) and have a section of a little bit more complicated licks so you´ll have to practice it do feel 100%

Some wah here and there is alwas cool too.

And when improvising a solo live for example, follow this structrure:

1 Mega slow (some kind of chord progression fits too)
2 Slow (like a nice melody with some modifications)
3 Medium (kind of 70´s rock solos)
4 Fast (somthing Rhoads/Hammett fast)
5 Mega fast (shred)

Petrucci usualy do it this way and it´s way more fun to listen than Yngwies solos wich are Mega Fast-Mega Faster.