Basic writing tips

English Grammar Online 4U is an excellent writing resource that provides a lot of very useful tips without getting too complicated.  Their General Information on Writing English Texts is a very good place to start improving your writing.

Advanced writing techniques for American university students

The Guide to Grammar and Writing is a resource for American college students so the language on the site is considerably more advanced than on a site for English learners.  However, the information is extensive and you may find the pages on punctuation (under Word and Sentence Level) and sentence combining skills (under Paragraph Level) particularly useful.