Read writers’ interviews in magazines like Writers’ News/Writing Magazine. These can be very encouraging and give a huge range of useful tips.
Never be afraid to edit!
Make sure presentation of material is good (no coffee stains etc) and that spelling and grammar are up to scratch.
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.
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.