Create Animations with Poser
First, you will want to download the SL avatar model files. These models can be used to generate more accurate animations for upload into SL. You can download the models from here. Installation instructions are in the zipped file. There are also example BVH files that may be of interest.
After you install the SL avatar figures in Poser you will want to select the male or female figure from the Library to use in your workspace.
To export from Poser, choose File/Export/BVH File. Save the file as a BVH file.
To import to SL, choose File/Upload Animation and select the BVH file you've just saved. The preview box gives you options for renaming, setting the priority, looping, hand positions, expressions, etc.
For users of Poser 7, the procedures will remain the same. If you decide to use the non-linear features of Poser 7 to animate, consider saving a Poser document of the animation as well for backup - exporting an animation to BVH will flatten it to a simple linear animation, making further NLA tweaks difficult.