In my experience, you don't have much to gain from negotiating; you may have a slight leeway to lower a monthly rate by just a little bit, but one thing you don't want to do (sorry aki, lol, gotta disagree with you here) is to imply that you'll have any sort of financial hardships or difficulty paying the rent whatsoever. Landlords don't want to hear that, they want to rent to someone who will be of as little trouble as possible.
They don't rent to someone who may not be able to pay his rent. Also, if you're too critical of the property, that might dissuade them too, since they don't want to rent to someone who they think might be calling them every day with issues. If it's between you and someone else, these factors may push them to that other person.
I'd say just look for places that are affordable to you, don't really plan on bartering. If in the process of negotiating you develop a good rapport with the landowner, then you can judge the situation and consider asking if a slightly lower rate is possible. Chances are that this can be done, but it won't be by much, so just look for places you can reasonably afford and consider any further lowering of the rate as a bonus.
But generally speaking, unless you're trying to rent in an undesirable area or you see that the property has been vacant for a long time, landlords don't usually have a lot of trouble renting these places out, so as a potential renter you don't have a ton of power.