Attempting to balance a redox reaction using half reactions leaves an unbalanced component I have this redox reaction that I can't balance using half reaction method. 

$$\ce{Mg + HNO3 -&gt; Mg(NO3)2 + N2O + H2O}$$

What I get at the end is 

$$\ce{2Mg + 6HNO3 -&gt; 2Mg(NO3)2 + 3N2O +3H2O}$$

I can't get the $\ce{N}$ balanced.
It seems likely that you are starting with the wrong half reactions.

Since this reaction happens in water, separate the ionic species into their ions:

$$\ce{Mg(NO3)2 -&gt; Mg^{2+} + 2NO3-}$$

$$\ce{HNO3 -&gt; H+ + NO3-}$$

Now, one half reaction should involve magnesium (and have no nitrogen anywhere). The other half reaction should involve nitrogen only.

$$\ce{Mg -&gt; Mg^{2+}}$$
$$\ce{NO3- -&gt; N2O}$$