Garry writes: 'This will sound strange, I am struggling at the moment with doing THE RUN, when doing an overhand shuffle. Sometimes I pull off two cards instead of one.'
Any sleight of hand is a battle! Without knowing the state of the deck you are using it is difficult to tell what is going on, it could be the pressure you are applying being too much. Try slowing everything down and being as smooth as you can. With the overhand shuffle it is always the goal to only use the amount of pressure you need for a move and no more.
A dirty overhand shuffle?
It may also be that he cards you are using have been bent or dropped and picked up dirt and dust, or even skin oils can sometimes affect the card's manipulability (I think I just made that word up!). Always make sure your hands are clean (I am not saying you are dirty!) and dry when practicing. You can always wipe over the cards with a dry cloth to remove any dirt, it sounds silly but grains of dirt (if the deck has been dropped on the floor) can get between cards and create more friction which will cause them to want to grip together, this is particularly apparent when fanning cards.
Dropping your playing cards
If you drop a deck of cards on the floor, try using a corner of a card to scoop them up so that you do not put pressure on them, rub them against the floor (they do not like that!) and bend them unnecessarily (they like that even less!). That is a great tip that will lengthen the lifespan of your cards
This might also help: 10 Tips to make your your playing cards last longer