Electoral roll
Credit card (used and paid off in full each month)
Mobile phone contract for a small monthly amount (not silly money)
Utility bills in your name
Landline/Broadband bill in your name
Sky/Virgin TV contract in your name
Basically, the more "evidence" that you're responsible with money - i.e. that you are taking out contracts etc for sensible amounts (normal amounts, not over stretching yourself) and paying on time.
I build my son's credit rating up whilst he was at University. Instead of us paying his mobile contract, we got him to take out a cheap deal (£10 per month) in his name, paid directly by direct debit from his student current account. Made sure he used his student current account authorised overdraft, but only a couple of times per year, for very small overdraft of £50 or so, and repaid quickly, incurring minimal interest. He put his own name on the student flat rental agreement, with rent paid by direct debit from his current account. As soon as he got a job, he applied for a credit card from that same bank - approved no problem at all, which he now uses for most purchases, fully paid off monthly. When the time came for "grown up" things like a proper flat rental agreement, home utility contracts, etc., we checked his credit score and it was very high, and now a few months after he moved into his flat, it's even higher!
It's all about "painting a picture" of being responsible with money.