Time and trophies is what it'll take. If you can stick with Bramhall Lane and start competing for and winning trophies then in 5 years time you could be sitting pretty. Attendances soon pick up after a trophy win and you've still got plenty of Sheffielders who remember your Challenge Cup success, so for a young club you have actually got a bit of history behind you too.
After Widnes were regulated by the formation of Super League our attendances nose-dived for a few years despite the construction of a new ground. It wasn't until we became competitive in the division that they started to pick up again to become the highest in the division and have yo-yo'd a bit depending on on the field success. A lot of fans felt (rightly or wrongly) that we had been cheated by the formation of Super League and drifted away. The farcical merge of your club with Huddersfield must've effected some of your fans in the same way (though you may correct me on this point as that is pure speculation).
If you can keep stable home ground that people enjoy going to and nab a couple of cups you'll surely get the attendances up, you've got a massive catchment area and your local soccer clubs don't do as well as ours so a bit o' success'll go a long way.
