Concentration of excess reactant
A very typical situation in organic chemistry is the following:

$\ce{A -&gt; undisired products}$
$\ce{A + B -&gt; target product}$

Here it is easy to see that increasing the concentration of $\ce{B}$, we will get a higher yield of the target product.
The other reaction can be of many kinds.
$\ce{A}$ can decompose in the reaction medium, can react with itself, with the solvent, with byproducts form the desired reaction, etc.