There isn't really that much extra you need to do. Further Maths uses the same style of question as the normal GCSE Maths exams and the questions are not that much harder than A* level in normal Maths. The best thing to do is complete as many papers as you can (our Maths dept. has loads but I don't know where they got them from).
Most of the content from the course is from C1 and C2 (although Matrices is from FP1 I think) so if you want a big challenge, complete exam-style questions from there. However, I should stress that the questions from the A Level maths course are significantly more difficult than L2 Further Maths. If you have no problems with the A Level questions, L2 Further Maths will be a breeze. Personally, I think the L2 FM qualification covers lots of the content from C1 and 2 but at a GCSE difficulty level, so it's a very good introduction.