One of the drawbacks is location. It's not difficult to put a few copies in your bag and hit the road around your local area, but getting it known further afield is more difficult. Friends and relatives who live across the country can do this for you but it's your book, not theirs, so their enthusiasm for trying to encourage people to show an interest in buying a copy isn't the same as yours.
When I approached smaller independent bookstores I was asked for an AIS, an Advanced Information Sheet, to be sent along with a letter of introduction. Of course, I didn't know what an AIS was, so on to Google and after a lot of hard work I finally came up with this. I then had some professionally printed, rather then running a few copies off at home, as the images look much better. I then selected 5 cities and started working my way through the independent bookstores and started mailing them out. As with any mailshot, I suspected the take up with be small, but it is worth trying. I also attach them to emails when I am enquiring about arranging school visits, as this gives the person I'm talking to a lot of instant information about me and the book, and it seems to work very well.
It is also something I used at a recent conference I attended. I just leave them lying around, even on the train home. You just never know what might get you a sale!