How can I balance the redox reaction of sulfur dioxide with oxygen? How do I balance this reaction using half-redox equations? 
$$\ce{2SO2 + O2-&gt;2SO3}$$

My work is:

Oxidation: $\ce{SO2 + H2O -&gt; SO3 + 2e- + 2H+}$

Reduction: $\ce{H2O + 2e- + O2 -&gt; SO3 + 2H+}$

Total: $\ce{SO2 + 2H2O + O2 -&gt; 2SO3 + 4H+}$

BUT it's false. What did I do wrong?
WELL I think you put something erroneous in your reduction formula. 

You will never get SO3 out of just O2 and water. Instead of SO3 

The reduction formla will be 4e- + O2 + 4H+ --> 2 H2O  

Multiply oxidation formula by 2 and add it to new reduction formula and you got your answer