First, follow the suggestions of the persons who have already responded to your question. After having done that, consider doing this. If you're talking about just a few rounds (100 or less), measure them all and then determine how you can get close to having all of them the same length.
If you're working with a large amount of brass and you have already determined the max length that you want to trim to, consider separating them into separate batches. I buy my brass in bulk; often by the thousand. I don't trim to length until they have been once fired. It's rarely much of a problem and I really have not discovered a huge difference in performance between the batches of cases that are a few thousandths longer, especially with the .308. I know that the guys who are chasing the lands in their rifles like that extra length but I have not found it necessary in my rifles.