Tip number one when using Bryce: Learn patience. Decent renders take time. I know, I don't exactly like waiting myself. When Bryce is done, it should tell you with a nice little popup message (if the preferences are set for it.) Stopping it before then is like opening an oven on something that's only half baked. Also note that some settings in the skylab's atmospheric controls can effect rendering time by a HUGE factor.

Use some lights (in addition to the default sun.) Yes it will take longer, but it's worth it if the scenes seem too dim. They do involve tweaking to get good results, even the experts will tell you that. If realism is the desired result, lighting is usually what makes or breaks a scene. Sometimes when outdoors, I've found that lights help to fill in what is otherwise too dark. (This is sorta like using reflector screens in real photography.)

After that, you'll need to check your render settings. If your settings are off, then sometimes Bryce acts funny. Also make sure that the anti-aliasing is on. It gets rid of the blockyness.

Also I recommend getting a 2D program for use with Bryce if you don't have one already. Bryce isn't so great at exporting JPEGs, so using another program to turn Bryce bitmaps to 'pegs will yeild better results. Also it comes in handy to postwork images in order to doctor the results that are usually too technically challenging to achieve otherwise.

Dunno how much that helps... But that's my $0.02

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.