When doing your taxes? If you trade on the fly and make hundreds of trades/flips every day for a year with equities options and futures. Do you spend a few weekends manually punching numbers or do you do your books by the end of everyday? How do you keep track of all your trades?
My accountant suggested a few years ago to choose Mark to Market: http://en.wikipedia.org/wiki/Mark-to-market_accounting I just give a list of all the trades to her, thousands of them, and that's about it.
For Sec 1256 futures (most of them) you just report your net PNL marked as of EOY. For stocks its a mess with wash sales and such, but you can just take your brokers 1099 and report that. I'd spot check any big gains, losses, or high notional value trades, especially if the company did anything unusual (split, merger, etc).
I'm using TradeLog, less than $250/year, unlimited number of trades. Support most brokers... (Selecting MTM, as someone else mentioned, prevents use of NOL's (Net Operating Losses), I believe). http://www.tradelogsoftware.com/ http://www.tradelogsoftware.com/support-center/broker-imports/