Most people are righty, so what I do is take my left arm, kinda chop at their elbow, and at the same time, use my right to grab that same side lapel and bring them close to me. Then that left hand I used to chop, I readjust and grab their lapel and keep them close to me keeping control of their other arm at the sleeve. Their first instinct is to create distance again by stiff arming and the moment you feel them straighten that arm out, pull them in for a seoi off the lapel. They would have given you enough room if they do that.
You can also, beat them to the punch and control that hand that stiff arms you at the lapel. Grab that sleeve, and push their arm to their body. From there I usually, grab their right lapel, and since I'm controlling their right hand(strong hand) they'll freak, try to control a lapel so they'll try to grab my lapel from their weak side and I'll go for a seoi.
You gotta learn to just hit throws off grip exchanges.