Viking said:1. Keep calm
2. Put and field dressing on the wound and keep pressure on it (aleays carry a field dressing on you) if you do not have a field dressing put your hand on the wound and keep it under pressure and then use a pice of clot or something that will keep a pressure on the wound.
3. Pick up your cell phone and if you are badly hurt call an ambulance and tell where you are or tell them to meet you on the nearest road or other place wher thy can get to. If the wound is not critical call someone you know and tell them what happend and where you are and what you are going to do (walk back stay the night). The make sure you call back every hour or so just to let them know you are ok.
I am with the viking on this, there was a mention of a field dressing being used, the issue field dressings are designed to be able to open them so that you do not contaminate the surface of the absorbant pad.
There is also a specific way that the dressing is bound onto the limb, you work the binding or bandage around the dressing so that you cover the dressing as well as just above and below the dressing itself, this gives compression over a larger area reducing the the blood flow and reducing the tourniquet effect (one area of compression which is narrow) as well as semi-sealing the edges of the dressing.
In all the dressing (which allows the blood to clot), with the compression provided by the bandageing on the dressing (slowing the blood flow) should do the job, but you can also lie down and elevate the limb above your heart level (rest it on your daysack or a log basically anything that will raise your leg by around 12 to 18 inches).
If it is not arterial, whilst you are down there put on a cup of tea and try to relax, if you smoke have a cigarette, it will help you relax and the tea will help to replace lost fluids and sugars.
If it continues to bleed heavilly get help, use your phone, whistle anything to attract attention to your location, but stay calm.