Uchikomi is a great training drill but if you are a bad partner you can actually annoy and restrict not only your partners learning but also your own. This is why it is so important to be a good training partner.
Where did Uchikomi come from?
I don't know if this is true but I got told that when judo first started no one ever did Uchikomi, participants only did nage komi (throwing). As judo became more popular there were a lot of participants who could not be thrown over and over as many could not breakfall. Then Uchikomi was invented and everyone could practice the breaking of balance and the entry without having to throw.
One problem with this approach is that many judoka have great breaking of balance and entry but due to them doing a lot of Uchikomi (and no nage komi) meant that they couldn't finish a lot of techniques in randori.
As a partner there are a few basic steps you should do to help be a better partner.
Firstly your partner must be a willing and competent Uchikomi partner. This involves being co-operative in all aspects of the drill. Your partner must understand that in order to be a good judoka, you must also be a good training partner. They need to understand that judo is like dancing, with an unco-ordinated, un-co operative partner many dance moves have no finesse and smoothness. This is the same as judo.
You must be confident in your breakfalling. If you are not confident of breakfalling you will not want to be thrown and therefore fight to evade every attack your opponent throws at you. Sooner or later you will injure yourself from trying to hard not to be thrown. If you are confident in your breakfalling skills you will not mind being thrown and will get injured nowhere near as often.
To be a competent Uchikomi partner you must firstly stand square. Don't stand with an extreme left or right stance. Do not stand bent over but instead stand in a nice upright position.
Take a grip
As an Uchikomi partner you must take a grip. Whenever you are fighting you and your opponent are always grabbing each others gi, therefore it is crucial that you attain a grip. There is nothing worse than doing Uchikomi on a partner who doesn't take a grip.
You must not stand there like a sack of potatoes. You must control your own bodyweight. You must never shift your weight or your hips when your opponent is performing their Uchikomi. Stand in the same positioning the whole time and let your partner control you by pulling you off balance and entering smoothly.
Don't look around
At judo you should always be concentrating on the task at hand- which is judo training. When your partner is doing Uchikomi look straight ahead and thing about the various aspects of the throw such as breaking of balance and the entry.
Have a plan
In most clubs Uchikomi is taken in turns, for example you perform twenty repetitions and your partner does twenty repetitions. There is nothing worse when you do your set and when it is your partners turn they take thirty seconds to think of a throw to do. This can be very annoying. To combat this when your opponent is doing their set, think about what you are going to do next. That way you don't waste anyone's time.
If you can implement all of these points consistently at training you will not only be a great partner but a pleasure to train with.