Something to keep in mind...
Actually, two things to keep in mind:
1) Ikkyo is a technique that takes a lifetime to master. Don't feel bad if you're having trouble with it.
2) I started out thinking about uke's arm/shoulder/elbow, but I find myself thinking and working more with uke's center instead of his extremities -- regardless of what part of his body I'm in contact with. Focus on having a unified body (hips turn, shoulders turn, arms move as one unit) and moving his center to his empty spot. That way, the technique will work regardless of the attack or the specific manner in which you execute the technique.