I've solved this in several ways that depend on the need.

1. Multiple small notebooks. I love ~50pg, 5"x7" lined notebooks. They're cheap, and portable. Just get one for each project.

2. I usually allow myself 1-2 extra pages for longer thoughts or things that will require follow-up. That almost always works.

3. Indexes. Set aside 20pgs at the front (or back) for a table of contents. I do this with my longer notebooks (~200pgs) and I never have trouble finding things. If you need to continue a section later on call it "Project X (cont.)"

I feel like #3 is something that generations before us knew implicitly that we've forgotten.

[edit: grammar]

[Edit: Addendum]

Also, always date your entries. That helps a lot.