My conventions are:

Fields:
No spaces on field names, EVER!
The PK of a table is always named ID (or the same for all tables), and when using it as a FK the convention is (tablename)ID (I write a lot of dynamic SQL and DMO, this makes things easier) - it also forces you to write readable SQL by using the (table).(field name) convention

Tables:
Use the tbl Prefix (tblCustomers) - very optional


SP:
Never preffix with sp_ (forces an extra trip thru the Master DB before resolving in current DB)