#1 Use an entertaining mission name:
Whan people make missions, [including me] they usually forget good mission names that makes them attractive. For example: you have a car race. There are tons of ways to name your mission:

Car Race
Drift Race
Impossible Race
etc..

#2 Make the mission nor short, nor long:
You are making a mission, and you are thinking about making a long mission. Keep the balance! Keep it normal (about 20-40 objectives) this will make your mission not to be boring because of the longness.
#3 Have fun designing!
If you are making missions day after day, day after day.. it's just so boring, that you want to kill yourself. Making missions should be fun, a hobby, not a daily routine.

#4 Make a topic in GTAForums:
Missions published in DYOM website wont get alot of attention. Create a topic in
Mission seaction request some GFX in GFX section, think of your own story, put in your GFX into the topic, add characters, trailer! Make the topic attractive, add tables. You can download table code generator here (Thanks to Barguast). Listen to people and try to improve!