Avoid complicated formatting and the use of large images that increase the page loading time. Also, it's better to send out a larger newsletter more infrequently than to send out smaller editions more infrequently (think spam filters.) The exception would be for time-sensitive materials, like leads or expiring deals.
Above all, respect people's decision to opt out. I can't tell you how many times I've clicked "unsubscribe" only to see the same newsletter cluttering my inbox every day.