How to answer writing questions?

I have a lot of problem with the questions that ask “Which choice most effectively combines the underlined sentences ?” in the writing and language section.
How should I answer them ? I only get them right half of the time at best .

Do you want to post a few examples of problems you didn’t understand? I’d be happy to explain them.

Reminder not to post questions and answers:
http://talk.qa.collegeconfidential.com/sat-act-tests-test-preparation/642156-defy-the-college-boards-rules-at-your-own-risk.html#latest

@“Erin’s Dad”
I believe that rule is for actual tests, no? Here we are just talking about practice tests, the answers to which are all public.