What the event will do for prospects, or what it will save them - more profit, improved career, less time or work. Don't reveal everything. Make them read on. Appropriate incentives always increase sales and profit by more than they cost. Get to the point.
Bad letters start with a couple of paragraphs of waffle.
The opening must talk about benefits as fast as possible. Faces increase attention enormously when placed at the top of your letters. Vital, especially at the opening. Don't try to be clever or funny. Pretty quickly you must get into a series of statements the reader can easily agree with - and keep them agreeing till the end.