Which of the following is the best way of preventing the development of deep vein thrombosis (DVT) in the postoperative period?
Patients are at increased risk for developing venous thromboembolism after:

Major abdominal surgery
Major orthopaedic surgery
Has sustained major trauma
Has prolonged immobility (>3 days)

These patients need some form of prophylaxis. The prophylaxis can be mechanical or pharmacological.
Prophylaxis for DVT 
Mechanical Prophylaxis include : 
Early ambulation 

It is the simplest method of prophylaxis
It acts by activating the calf pump mechanism

Pneumatic compression devices

It is the most common method of prophylaxis
Pneumatic compresses prevents the stasis by periodically compressing the calves.

Pharmacological Prophylaxis include

Use of unfractionated heparin - Now being replaced by fractionated low molecular weight heparin
Use of fractionated low molecular - Better efficacy than unfractionated heparin
weight heparin - No laboratory monitoring is necessary

Comparison of low molecular weight heparin (LMWH) with mechanical prophylaxis demonstrates superiority of low molecular weight heparin (LMWH) in reduction of the development of venous thromboembolic disease.  Prospective trials evaluating LMWH in head injured and trauma patients have also proven the safety of LMWH with no increase in intracranial bleeding or major bleeding at other sites. In addition, LMWH shows significant reduction in the development of venous thromboembolism compared to other methods.
In short, LMWH should be considered the optimal method of prophylaxis in moderate and high risk patients. Even the traditional reluctance to use heparin in high risk groups such as the multiply injured trauma patient and the injured patient must be reexamined, given the efficacy and safety profile of LMWH in multiple prospective trials.
Early ambulation
Physiotherapy
Prophylactic heparin
Low dose aspirin