There are a couple of things that you can do to stem blood flow from a wound:
Keep pressure on it.
Keep it dry (water acts as an anticoagulant, stopping the blood from clotting and eventually forming a scab).
Keep it raised above the level of your heart (your heart is what causes blood to pump around your body, so being "uphill" will make the flow slow down.
Use a tourniquet to control heavy bleeding to a limb. This should be used with extreme caution, do not leave a tourniquet on for extended periods of time. Keep loosening it to check on the bleeding.
If this is a major bleed, I would recommend contacting the emergency services at once.
Well you can avoid it all together by not getting cut or injured. You can also stop bleeding if you freeze someone, so when you cut them they won't bleed that is until they defrost but usually the blood will have coagulated by then!
