Be positive! elbowfan has loads of good points. But don't ram it down their throats, I hated that when viewing and the owner couldn't give you 2 seconds without banging on about all positives.
Our house was on the market 5 months, loads of viewings, not a single second viewing (despite good area, good school 4 doors down road which fed into great secondary, etc etc) but the house did 'need some updating' ;-) . We let the estate agents do all viewings on our house, but crossed over with them on one, and I don't think they were doing such a great job- in fact, i KNOW i could have sold it better but was pg, and working full time, and not able to.
And incidentally, when we sold our house (well, part exchanged), the guy wandered round for 15 minutes looking not particlaury enraptured, said 'god, they liked woodchip a lot' (too true), 'it's not as loud as I thought it would be' (main road location), 'how many bedrooms is it?' !!!, followed by, 'ok, full asking for yours, full asking for mine?' stuck his hand out and shook on it and that was that!!!! We had also viewed his once for about 15 min.... And I was totally despondent about ever moving by that point.
We were in our new house 6 weeks and 4 days afte that.
The point being, you never know who's going to pop up. And I did have the breadmaker on. Both times he came. And certain windows open. And only EVER mentioned positive things... Not schools as he was going to rent it out, but alterations like new doors, recently serviced boiler etc etc