How do I improve navigation with a VFR sectional chart?
The trick is not to fixate on small details, but to scan on the chart along your route for landmarks and features that combine into large geometric shapes that are easy to pick out from the air (while cruising the higher the better).   

Like; highway, river, and railway line that form some shape like a large 5 mile long triangle so that there is a unique pattern, with a town at some point on the triangle that makes it especially distinctive, to the left of your track.  

Note that configuration on the map.  Try to mentally compress it in your mind to allow for foreshortening because you will be initially viewing it at an angle.  Look for its match on the ground, allowing for its location to be farther left or right than expected if you've drifted off track.  As you find it and know where you are, scan ahead on the map for another easy to spot shape, as close as possible to your current position, note its distance, estimate the time to get there based on your ground speed and wait for it to appear. 

Beyond that, it's mostly about establishing the correct heading to stay on track and holding it well, staying on top of your position and not allowing distractions to divert your attention too long as the world slides by below, and getting good at mentally projecting your progress based on known ground speed (like, you've been distracted for 5 minutes since your last known position, and if your ground speed is 2 miles per minute you should be between 9 and 11 miles away from it, so start looking for a recognizable geometric pattern ahead of you, say, 12 to 15 miles from last known).

A home computer flight sim like FSX or Xplane is really great for practicing this sort of thing on your own time.