Ian,
First of all, like BlueNote says, I would learn the dominant scales/arpeggios really, really well. If you look at almost any jazz standard, it has a significant number of dominant chords in it, so this is a really useful sound to know well. A useful book at your stage of development might be "Gettin' it together", published by the Aebersold outfit. It's got some great tracks that you can practice your scales/chords with, and its fun.
After you 've got the dominants under your belt, there are really 2 different tracks you can then proceed on (or, you can do both :lol: ). One is to get "Patterns for Jazz" as noted above. I have it and it's a very good book. However, (in my opinion only) the "pattern" approach, while teaching you to play some really cool licks, doesn't really help you learn to play smooth, flowing, musical lines as you go through the changes on a real tune. Personally, I would learn cool licks as an adjunct to another approach, not as the primary approach.
Another approach is to get a book called "Intermediate Jazz Improvisation" by George Bouchard. This very thick book is really a sophisticated "lesson plan" to take you to the next level, as if you had a teacher who really understood jazz and how to teach a developing player. The book assumes, I think, that you are proficient on major, dorian minor and dominant scales/arpeggios; if you don't first have these under your fingers, I think the book might be too frustrating.
In my opinion, there are two aspects to practicing: first you have to be willing to consistently work hard over a long period of time; if there's a shortcut in this sense, I'd love to hear about it. Second, you want to make your practicing as efficient as possible so you get the most "bang for your buck" for all that effort. That's why a book like the one above can be so valuable; it can keep you directed and focused, so that you progress more rapidly.
Hope this helps,
Al