if you know what you want to buy and don't want to trade very often, online is a good choice.  TDameritrade or scotttrade for example are not very expensive.  you pay about $10 when buy or sell.  no other charges. 

1)You can buy ETF using cost average method...It only makes sense if you have free trade like BOA offers. Otherwise, commissin fee could add up.

2) or you can buy index fund with automatic investment choice..look for the ones that have low expense fees. Vangard usually offers good chices.