I know you said: "anyone got any tips apart from not offering large plans?" but I would like to suggest that some large plans are ok, but work up to them.
If you are P2H, well then that wouldnt apply I guess, but if people are going to need X amount of space and X amount of transfer, then they should have some way of getting that.
I would like to use exphost as an example (hopefully Stu wont mind). He offers up to I think 10GB/100GB, but you start off with far less and upgrades as needed. Say you start people with 200mb/3gb and then expand as people need it. I think this would show people that you are willing to work with them, but more experienced webmasters will know that you arent catering to the warez hopefuls right off the bat with a huge offer.
Another tip, support. Any host whether free or paid should have timely support by individuals capable of handling questions. If you start to have a large client base, get more people capable of handling cases, but responsible enough to do so on a voluntary basis and not stop by once a month. Even a reply such as "This topic is out of my expertise, please be patient as I submit this to whomever I need to" or along those lines. Having a support section in your forum that goes unanswered for weeks on end will not keep clients and will likely dissuade people from joining if they peruse the forum and see issues are not handled. People will state "You get this for free, dont complain" my response if if you offer services, you should still be professional enough to take care of clients, ESPECIALLY if you require another form of 'payment' such as ads or posting. Instead of giving money, they are giving something much more valuable: time, and that can never be replaced.
One last thing I can think of off the top of my head: Never put the "99.9% uptime guarantee". Should you fall below that, there is nothing a free host can logically offer as remuneration. More space/transfer for one that doesnt need it is pointless, and you certainly cannot give money back/add a month since they never paid for it, unless you are P2H, where you can give a month of free posting or whatever.