STAAR Online Practice sites:
Prepdog: A site created by teachers. It has practice questions for all STAAR subjects. The formatting is different from STAAR, but (for reading and writing at least) it has good questions.
pearsonlongman.com: It has two pretty good reading practice passages.
Reading Sage: This is actually a site for the California State test, but it has some good questions. Not all of the writing ones are over what the Texas writing test covers, so stick to the ones on conventions.