Make sure the letter is not longer than one page, it isn’t too long, has good margins and lots of white space. Make a good first impression.
Punctuation and grammar
Can you write properly? Don’t be to sure. First be careful yourself, then get it proofread by someone else!
Addressed to a person
Always useful if you have a name to send the letter to, it makes everything clearer. If you don’t have that luxury, then perhaps you should simply start with a reference line as the opening, for example – Ref: Position as Bioengineer advertised in the Who Paper, January 1999.
Sign your name
Good thing to do, if possible.
No funny business
No pink paper, strange typeface or other attention getters that don’t work. Unless there is a particular reason to sending a portfolio or other creative endeavour, where you think it would make a difference.