How to thicken marinara sauce?
The best way to thicken marinara sauce for me, without losing any taste is to cook it a little longer. 

Cooking it longer is just keeping the sauce on simmer, uncovered and stirring it occasionally so its cooking consistently and taking it off the heat when you think it has reached desired thickness.

You can also try draining the tomatoes before you crush them. 

You could even try adding little breadcrumbs, but it might change the taste.