Here are some resources to help you improve your formal letter writing skills in preparation for the Cambridge Advanced exam.
This first video gives general tips for the writing section of Cambridge Exams:
This video explains the differences in formal and informal language used in letter writing.
Flo-Joe has a series of video lessons on formal letter writing:
This link from EngVID is a handy reference of informal language and its formal equivalent: ENGVID FORMAL/INFORMAL
Need help with formal expressions? Click here: FORMAL EXPRESSIONS