When a reaction is endothermic just place a little triangle symbolizing heat on the reactants side and treat it just like a reactant. So if we increased that reactant (heat) then it would drive the equilibrium to the right. Now think of an exothermic reaction, place the triangle on the right as a product, if we were to add heat to the reaction mixture we would be adding a product and driving the reaction back to the left. What if we took heat out of the reaction mixture and the reaction is exomthermic, we would then be removing a product and pushing the equilibrium to the right. If this doesn't make sense just say so and I will try and find another way to explain but you should try writing this out to see if it makes sense.
Its all about maintaining equilibrium. If you have the equation "A+B <==> C+heat" the reaction from right to left is obviously endothermic and left to right is exothermic. When you add heat it will shift the equation left, which is clearly the endothermic direction. Its the same principle as adding more C which which also shift the equation left.