First off, some amount of stress is good. Stress isn't always bad - there are different types of stress, and different people react differently to it.
Also, stress will always be unavoidable - if it isn't, it means the team isn't working.
That being said, a good team leader can do a lot to help. Often, it's more a matter of pairing the right developers in the right parts of a project, as well as making sure the team has the right breadth of experience and interests to fill the project space well.
If you pair people up well, you're 3/4ths of the way there. As a crude example, don't put somebody who loves working with databases into the role of designing a UI, etc. This will make people more productive, but also happier. (It's obvious, but I've seen this abused a LOT).
Also, praise and enthusiasm from management goes a long way, too. People will get stressed, if they feel appriciated, they'll get through it easier.
From a personal note, I find that regular excersize, and personal rewards go a long way to helping me manage my own stress. Also, if I know I'm going to go reward myself with a nice dinner, or a weekend ski getaway, etc, after finishing a big project, it makes it easier to get through the project when I'm stressed out.