Read writers’ interviews in magazines like Writers’ News/Writing Magazine. These can be very encouraging and give a huge range of useful tips.
Editing
Never be afraid to edit!
Presentation
Make sure presentation of material is good (no coffee stains etc) and that spelling and grammar are up to scratch.
Multiple Submissions
Try to have more than one piece of work “out there” at any one time. It can help cheer you up when the rejections come in knowing you’ve still got work under consideration.
Write More
The more you write, the more pieces you can get out there! Don’t just wait for one piece to be accepted before you try anything else.
The Importance of Being a Writer with a Decent Dictionary on standby
Always have a good dictionary on standby.