A good habit to get into is to avoid zoning with the paint tool, always use the Marquee tool and zone one block at a time. 2x3, 3x4, 4x5, 2x4, 3x5, 4x6, etc. Only zone that density for which you have a full bar, don't even bother with a bar that's only half full. Keep an eye on the demand bar after each block you make, because if it IMMEDIATELY goes to 0 after a single building got zoned, then that's all you needed to do. Eventually it will replenish and then you can zone another.
Also use the marquee tool to sprinkle in office and commercial near your medium and high density buildings, because your businesses will get the "Not Enough Customers" flag if you decide to zone a commercial complex too far away from your residences. That's right, customers are a resource. So you have to zone commercial where the "deposits" of customers is highest. XD
If your citizens are well educated and they live in high density blocks, placing office buildings near their apartment buildings will cut down their commute and allow them to simply walk to work. This increases the value of their apartments, the offices will be happy because they have a steady supply of labor, and you won't have to deal with nearly as much traffic.