A few tricks I know of are as follows. We'll start with the obvious, never establish them at an angle greater than 30 degrees. Have an idea which altitude they should be at for their distance from the field. I usually do 3000ft and stablish em at 10dme but when things get busy that doesn't work so look at a few charts and look at the descent planning part but if its anywhere over 13dme I give it 4000ft and anything above 15 I give 5000ft. Also, take into account the A/C types, for example you would give an atr much less turning space than an a380.
Also, if there are waypoints in this simulation in the right places, take a look to see if you can get an aircraft to leave waypoint XXXXX on a heading which it can establish frrom. Ie for Dublin, aircraft coming in on the L975 to LIFFY, get themt o leave LIFFY at hdg 260 and they establishe nicely just outside 10 miles, now I'm awarre you're probably not sure of ythat airspace but it's a good example.
Hope that helps you.
DK.