For close-up trees, get leaf photos, as Erlik says above. For trees you are going to put in the middle-distance or distance, try using some photos of tree foliage to make picture textures, using parametric or random settings. This works especially well for trees which are meant to be large, as it breaks down the leaf outline and gives the impression of smaller leaves, and more of them. you also get a variation in leaf colour, which helps with the realism. For the trees themselves, you do need to experiment with the settings. You can also get some interesting shapes by skewing them with the Resize controls. For mature trees, try combining two or more slightly different trees together - you get more branches, and you can add branches where Bryce won't put them, such as low on the trunk, or to make an asymmetrical crown. Bryce trees can actually look quite good, and I think it's a lot of fun experimenting with them. It does take time to learn what works and what doesn't, though.