You need a benchcapper. Do not use a hand held capper - it will break the tops off.
I have been using twist offs for bottles for years and have never had a problem with either the beers being flat due to the co2 escaping nor have I had a bottle break whilst capping. Use a benchcapper though!
Primarys : Ginger Beer, Wheat Beer. Secondary : Empty Bottled : all drunk Drinking : A Lot. Next Up : Wheat Beer with grainy things