As LtL says triceps can play a part in it, but also lats and shoulder flexibility.
I did it this way:
1. Taking a empty barbell or a broomstick
2. Get it in the clean position (even if it not touching your shoulders)
3. Have a buddy press your elbows UP and/or the bar DOWN. To get it in position, hold there for 30-60s for multiple sets.
Painfull as hell, but it works.
Also powerclean alot and try to catch the bar in the clean-rack, one day you suceed =)