i think u will find that the main focus of the sdd exam is on scenarios, ticketing, railway system, etc. and u have to apply ur knowledge to the scenarios they throw at u.
best practice would be past papers and similar questions with worked solutions. learn the style of answering each question, this would include going back and revising the content(for eg- dfd's, feasibility study, ect)
anyways thats my 50 cents.