How do I reduce how much the sun heats up a front room of my house?
A good deciduous tree helps, but it will take a while for that to work.  Just be sure not to plant it too close to the foundation.

Light blocking blinds are very good at blocking both heat and light.  This may not be good if you're trying to work in the room.

Insulation helps if you have an attic above, just don't block the ventilation from the soffits.

However, the important thing to realize is that cooling systems spend a significant effort overcoming the heating effects of people themselves.  And a couple computers doesn't help.  No matter how much you block the outside heat, the heat being generated is still there.  Therefore, make sure your vents are adjusted to cool the rooms you use (closing doors also helps), and direct most of your cooling to the upstairs.  If that still isn't enough, try a small fan in the doorway blowing in cool air from the floor (which will push out the hot air above).



Edit: Couple other thoughts.

Get an IR thermometer to check all your walls and various other heat sources.  You may discover a patch of uninsulated wall or find that the window it letting in more than you thought.

Second, check your thermostat to see if it has a setting for how much the temperature will vary before it cuts on.  Mine is an odd key combination that's buried in the back of the manual.  If you're going well over an hour between AC cycles, you'll reduce hot spots by reducing this setting.