I panfry the steak by searing on high both sides, then cook it at a low-medium temperature until medium rare. Then I let the steak rest for ~10 minutes before cutting it. During that time a lot of juice leaks out. What am I doing wrong?
A lot of people had good questions in the comments. The best being Kenji's article from serious eats .com. Read that.
Chances are if a lot of juices are coming out of your steak, its because you are cooking it past medium. I like to think the proportion of 'grey' meat in a cooked steak indicates how much red juice has been 'squeezed' out of the meat fibrils and is now on the plate. If your steak is still medium or medium rare, there is lots of 'red' meat that has the capacity to hold the juice! -Most important: Let rest ten minutes (longer for bigger cuts/roasts etc) after cooking. Seems like you're doing that.
Heres one link to a Serious Eats article by Kenji. This one explains some myths, including 'locking in juices'.
This is normal. Heating meat proteins makes them contract, which squeezes out water which was previously happily trapped between them. You can't avoid some of this happening at any temperature - the only way to not lose any moisture at all is to not cook the meat.
As you sear it, the outside obviously gets very hot, the water boils out and the delicious browning happens. The interior begins to warm, and as you reduce the heat and then as you rest it it's still rising in temperature up to the point you want to serve it. All that time the proteins are contracting - quite gently, if you're cooking it to, say, medium-rare - and water will come out. Just less than would if you went to well done all the way through!
If you're making a sauce to go with your steak, put the juices from resting it into the sauce for extra beefy flavour. Assuming it's that kind of sauce anyway.
Unless the steak seems dry or tough you're not doing anything wrong at all. There will always be some moisture loss and you simply can't avoid it - but you can minimise it within the boundaries of how you like your steak cooked.
Let your steak sit out until room temperature. Pre-heat your oven to 350°F. Pre-heat your frying pan to searing hot. Pat dry your steak with paper towel and rub each side with salt. Put the steak in the pan and sear each side for 1m. Put the steak into the oven for about 5m (depending on thickness). Let it rest on the counter for 10m.
An alternative which keeps 100% of the juice is to cook it sous vide. That might be too juicy for you though!