The best advice I could give any aspiring or established author when submitting their manuscripts is to do their homework. Rather than send out a blanket letter with their submission, they need to research each publisher's submission criteria. For example, some publishers/agents request the first/any three chapters, others like the first 30 pages, others need a submission form to be completed, and so on. If you submit your manuscript without doing your homework then you are likely to be rejected on this basis alone.
You also maximize your chances of being noticed if your manuscript has been professionally edited/proofread.
I actively solicit other authors and those working in publishing for suggestions on how writers can improve their craft and/or get published. "Be professional" is a very common refrain.
Anybody with a writing tip is welcome to email it to firstname.lastname@example.org and I do publish nearly every comment, even those I don't necessarily follow in my own writing. I sometimes combine pro and con viewpoints into a single post, or put together a series of very brief tips on the same subject. I reserve the right to do minor editing because, well, that's just the way I am.