In the couple days prior to starting a new rotation, take a little time to do some light reading regarding the core of the rotation you are about to start. If you are doing OB/Gyn, for example and you know you start on OB, read about the labor/delivery process. If you start gyn, review the STD section. You might want to look at your pelvic female anatomy as well (lots of hysterectomies).
If you know you will start surgery on a vascular service, for example, you might want to give a light read of the vascular surgery chapter in your text first. For peds, review your developmental stages.
Then spend the first day or so getting familiar with the things unique to that service that need to go in your notes. Eg for OB/gyn the G's and P's and how to use the pregancy wheel is important. For surgery (and pts with gyn surgery) post op day, wound checks, and in's and outs broken down into urine and each tube/drain the pt has (listed individually) is important.
Once you've got these things down, start reading based on the pts you've seen. Then start filling in with the major areas of the field that you haven't seen.
Learning clinical thinking is a process, which will continue to evolve throughout your training. Some things are fairly easy (eg replace electrolytes when they are low). Some things are a lot harder (like the timing of an operation in a pt with multiple medical problems). And for a lot if it there is no single right answer. You'll find what two different attendings may do in the same situation varies somewhat.
Most of all, try to find some value in each rotation, even if you dislike the field in general. Each specialty has something worthwhile for every physician.
