Seoi Nag ia a favourite throw of many, as it is that rarest of things in martial arts: Something that not only looks great when done well, but is also highly effective (when done properly). It is very tough to do in randori.
My own personal favourite tactic was one I learnt for Kata believe it or not. This is why I valued Kata in Judo so much; apart from giving you a good grasp of basic Judo and its principles, it also gave you different ways to approach randori.
When performing Uki Otoshi, first part of Nage no Kata, I noticed how effective the drawing down phase of the kata was i.e. were Uke stepped back and pulled Tori down by bending his/her knee. Kind of like a lunge in reverse.
When I did this in Randori, I noticed that my oppoenent always presented one of their feet to me, usually on the side that I bent my knee on. As I as lower to the ground than they were, it was easy for me to grab their heel (having released my grip from their gi sleeve), pulling their heel as hard as I could while pushing their upper body with my other hand (that had remained in the lapel of the gi).
Couple of points to make about this:
i) Do let either of your knees touch the ground when drawing back and down.
ii) Without question, you must be quick to do this (as with most throws, really).
iii)If you get familiar with this, then feign with a supine technique like Sumi Gaeshi when you first draw back, then quickly draw back again (if possible).
Just a little tactic I came up with, all thanks to Kata.
I would also recommend you do work on combo throws. If you are going for Seoi Nage and missing, yet getting half way in, why not switch to say, Ashi Guruma instead? Judo is about speed of mind as much as speed of body.
A common mistake for beginners in Judo is to pick their favourite throw and "go looking" for it during Randori. They are giving themselve a lot of trouble.
It's better to have few different things up your sleeve.
"Let your food be your medicine, and your medicine be your food"