Continuous Intensity Repetitions (CIR) bouldering, from The Self-Coached Climber, is one of the most best methods I have found for improving my climbing. CIR is climbing a set of problems at particular, sub-maximal grade with near complete rest between problems. Lately, my CIR sessions are ~12 problems at V6/7 and last ~45 minutes.
Why bouldering? Distilled, focused movement. I can visualize and perfectly execute short sections of climbing. Why 12 problems? It gives me a breath of movement opportunities while maintaining precise execution. If I climb more problems, I get sloppy. I don't want to practice, thereby ingraining, sloppy climbing. Why that grade? It is the grade I can consistently send 1st try. Why near complete rest? When I get pumped I flop around like mackerel in the hull of fishing boat. I want the movement to be brilliant, but balanced with the entire session not taking too long.
When I convince other boulders try CIR there is an immediate improvement in their climbing (and their enjoyment of climbing). That could be the Novice Effect, aka something new eliciting an improvement. I think it goes deeper. CIR provides a platform to increase your movement repertoire. It tricks you into climbing more than a typical bouldering session of projecting limit-level problems for far too long.
I use the following variations of CIR to accomplish different goals:
Onsight Only: Usually done at visiting gyms or crags. A great way to sample an area while getting a training dose.
All Different, But Repeats: Exposes subtle movement limiters.
Circuits: I pick a set of 4-6 problems. Climb each one, and repeat the entire set of problems 2-3 times.
Efficiency Grinders: I pick a problem and climb it 3-4 times in a row. I repeat that process for 4-6 different problems. I refine each repetition either by experimenting with different sequences or becoming more efficient with the same sequence.