The best way to maintain/raise your lexile number is to read a variety of literature. I would suggest reading books that are of a higher reading level than your current grade (take a look at your schools reading list for juniors or seniors). Also make sure you read not only fiction, but non-fiction(bios, newspapers, etc.) as well.

Here is a site that actually lists books by lexile number if you want to know where to start:

http://www.lexile.com/findabook/StudentInfo.aspx

Just put in your lexile number on the first page, choose the genre you would like to search, and the site will automatically generate a list of books that are at your lexile level.