The best way to improve your reaction papers is ... to write reaction papers, and of course to work on the particular issues I mention while grading them.  Still, there are several general tips I can also give you.  They fall into two broad categories:

A.  Better preparation.

1.  Do the reading assignment.  This isn't exactly a profound suggestion, but I might as well mention it. 

2.  Read it again.  You'd be surprised what a difference it makes to go through a text twice:  really fast the first time, just to get the lay of the land, and then more slowly the second. 

3.  Graffiti.  Margins are made to be scribbled in.  Star things that look important; ask questions or make objections; restate things in your own words; divide the text into sections; make an index of topics you'd like to keep track of.  Do these things as you read, and you'll be much better prepared to write something thoughtful.

4.  Discussion Questions.  Try writing a few of your discussion questions on the text that pertains to the RP.  That can you come up with some unusual insights to use in your writing.

B.  Better writing.

1.  Stay on topic.  Always a good idea, and especially important when you have limited space to write.  If I ask you to explain what Socrates says justice is, and instead you tell me your own view of justice, I can't give your paper a good grade -- even if it's otherwise brilliant.

2.  Mention specifics in the text.  The best RP's stay clear of vague generalities.  Instead they dig into the reading assignment, giving concrete examples and using specific passages. 

3.  Summary vs. Analysis.  At the same time, giving lots of specifics isn't enough, either.  If you only summarize what comes up in the text -- this happens, then that happens, etc. -- you aren't really doing much to understand it.  Generally, if you do a good job staying on topic, you'll also manage to avoid mere summary.

4.  References and quotations.  This is perhaps the easiest change you can make to improve your RP's.  If you have drawn an idea from the reading, tell me exactly where it came from.  Use parenthetical references in the proper form.