A drop in online trading from 7.2%of American households in 2000 to5.9% this year is causing headaches fordiscount stockbrokers looking to shoreup faltering revenues. As online tradingvolume wanes, brokers are fighting formarket share. Some, like CharlesSchwab and E*Trade, are rolling outcredit cards that award points that canbe used for free stock trades. For now,however, the most popular businessretention tool is a deeper discount foractive traders. For example, if youmake 27 trades or more in a quarter,you can cut the commission at E*Tradefrom $19.99 plus a $3 handling fee pertrade to just $9.99. Over at FidelityInvestments, $30,000 and 120 trades ayear will get you an $8 per-trade commissioninstead of the usual $19.95.