Selling Losing Shares vs Winning Ones

[mtgordon]

I have shares of a stock that I've purchased over the past year. Some of these shares are currently up while others are down in value. In order to avoid paying taxes on selling part of the stock (at least for now) I would like to sell the shares that are down in value. Is there a way to do that (especially on Zecco) or do I just sell and write it up as a loss or what?