1. A lot of luck. Someone can win one series who would have had no chance in another series. Brian Dowling is the only housemate I think would have won any series, and even that is not certain.
2. Don't get caught up in a 'nasty group'. You can either stay out or be a target, but you can't be one of the 'coven' of 'bullies'.
3. It probably helps to be quite good looking; you certainly don't have to be dazzling, but very plain looks seem to tell against someone.
4. You have to appeal to women, since they appear to be the majority of voters. Women (not surprisingly) don't tend to warm to women who are bitchy, predatory, or appear to look down on other women.