What is the most effective way to extract mint flavor for a Mojito?
The best way I've found to get full flavor out of the mint is to infuse your liquor with it. I do this for mint juleps. Simply take a handful of mint, bruise it (you can just crush it in your hand or stick it in a bag and whack it a couple of times with a wooden spoon), and place it in your liquor of choice (white rum in this case) and leave it for a day or two. You'll need to experiment with the right amount for your drink, but I loosely pack a mason jar about half full to do bourbon. 

If you want the actual leaves in your drink, then you need to make sure you muddle the mint rather than just putting it in. What you want from the mint is the oils in it, these are released better when you bruise the leaves. So just chopping and throwing it in won't work well. You actually need to crush them. 

It's also important to use the lime rind in addition to the juice. And again, muddle it. The reasoning is the same. There are oils in the rind that are released during muddling that have a decidedly different flavor than just the juice. 

To finish out, pick your sweetener. I like to use simple syrup, but you can also use sugar (or anything sweet, if you want to change the flavor, try a flavored syrup). And then top it up with something non-alcoholic. I particularly like sprite, but also use lemonade or sometimes just soda water. Be careful to balance the sweetener based on whatever you're adding; Sprite obviously needs less sugar added than soda water.