Soloing does require you to make the leap into creative playing on your own- study materials and a pure intellectual approach will not get you there. We're assuming here that you're reasonably comfortable playing general solo stuff (even if very simple) in time and without losing your place in the measure. If you're not, see the tip at the end of the piece, and/or start getting comfortable improvising with very simple materials: quarter notes, quarter rests, 8th notes, and 8th rests.
Get comfortable with generic four and eight measure phrases. At first you should be able to just play for that many measures without getting lost, maybe with a crash at the beginning; soon you should be able to put a more musical ending on each section, and set up the next section. The goal is to make a coherent melodic phrase, with a conversational arc, just as if you were telling someone something interesting vocally for an equal length of time. If you experience problems keeping track, start with two-measure phrases. It will also help (and is good practice musically) to restrict yourself to one or maybe two ideas per phrase.
Much more after the break:
Learn common forms in broad strokes, in the process learning to think in terms of themes, repeated themes, contrasting sections, and recycling back to the beginning. Each of the following forms is used for many, many tunes:
- A 12 bar blues is a 4+4+4 measure AAB. A theme is played twice, plus a turnaround bringing you back around to the top of the form. Coltrane Plays The Blues is an excellent album for getting this shape burned into your memory.
- I Got Rhythm/"Rhythm changes" is a 32 bar (8+8+8+8) AABA- the B section (the bridge) is distinctly contrasting, and the end of the last A should bring you around to the top.
- All The Things You Are is 36 measures long: AABA with a 4-bar ending tag (8+8+8+8+4).
- All of Me is a 32-bar AB (16+16). Each section states the same theme with a different ending, so it could also be called ABAC ([8+8]+[8+8]), with B and C sections as endings, rather than distinct sections.
Learn the tune. Whatever particular tune you happen to be working on. Play the rhythm of the melody on the snare drum, preferably while singing it, even if badly. Pay attention not only to the shape of the melody, but also the negative shape- be aware of where the breaks are. Note anything going on with the arrangement- stops, ensemble figures, written dynamics, etc. You should be aware of the harmonic rhythm (the rate at which the chord changes happen), though you may only be able to use this intuitively at first. With a little creative re-voicing, some Ted Reed/Syncopation-style filler in the melodic rhythm, and/or fills during the gaps in the melody, you will have the beginnings of a musical-sounding chorus of solo drums.
Here's a tip if you're at a loss for what to play: take an mp3 recording of the tune and make a practice loop of the melody (using Audacity), then run your practice materials- anything at all- along with it. You'll be surprised at how "together" the written materials are with the tune, just by accident. Hearing the exercises in the context of a tune will be the key to introducing them into your playing, if you're taking the right approach of only performing things that you hear as music (not just things you practiced).
Jon McCaslin @ Four on the Floor has an excellent post which I've linked to before on this subject- this would be a good time to reread it...