Scott .. the bible says 'to flee youthful lust'. How can you flee youthful lusts living together? Also the bible says to avoid the appearance of evil. When people see you living together, the automatic assumption is you're also sleeping together. So you're placing yourself in a position of 24/7 temptation and giving others the appearance of pre-marital sex. I'd say thats wrong.
And seriously, how would you know how strong you are in resisting unless you're put in the situation? You have to put yourself in a situation thats set up for you to fail in order to really know. So what happens when you find out you don't have the strength you thought you did? Too late now. You've slipped up, lost your purity, let down God, let down yourself, your future mate, will have to cope with the guilt, the possibility of pregnancy. Live together... great advice.
Mad .. other than not to engage in any explicit sexual behavior, sex, foreplay, oral, touching 'private' areas, etc... There is no rule. Some people feel kissing is a sin, some people think making out is ok. Some people think hand holding is too much, others love to snuggle up on the couch. Its largely up to the individual, but i suggest playing it on the safe side. Bottom line is the more you allow, the more you risk. Don't try to define the line, then live as close to the edge as possible, but define the line for you personally, then make a choice to do a little less than that, so you give yourself some space so that if you do go a bit farther on accident, you don't actually cross your own line. Err on the side of caution, not with getting away with as much as you can.