Keep the crossbar straight. It looks bad when its curved and even worse bent at right angles. This illustration is from the Unicode Standard version 1.0, volume 1. Later editions, I am assured, will show a better example.
No way out
A lower case eth shouldnt be connected to the letter that follows. In script designs, an exit stroke is out of place.
Step by step
In the typeface Park Avenue, the letter eth, left, used to look bizarre. Fortunately, it has been changed.
The current version, second from the left, still leaves room for improvement. Moving the crossbar down, as in the second letter from right, would help. And a well-shaped ascender, right, would make a decent letter.
Needs work, that eth
The example on the left shows what can befall the letter eth when misunderstanding and imagination combine forces. It used to be a part of the typeface Brush Script. The middle letter is its current version, with an exit stroke and a bad ascender curve. As the letter on the right shows, it can be mended.