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I typed a nice, detailed reply to your question and Yuku ate it because I wasn't logged in. Here is the 3 sentence summary:

1. Pick good quality mare that has all the characteristics you want your mule to have.
2. Pick a jack that has a good disposition, a successful performance record, a history of siring good mules, and one that, as mulelisa said, lacks prepotency (i.e. his mule babies aren't donkey-ish).
3. I bred an athletic, leggy Clydesdale mare with a great disposition to a large standard jack with all of the characteristics listed in 2. and now have a fantastic young mule with great potential who is good sized (16 hands) but not drafty.