01 Apr 2009 at 23:57 - 10
Please read the stickied guide for basic information about secondaries. Some of the information is outdated, however it provides an excellent foundation for you to explore your options. The assassin has many secondary choices which are viable, your best bet is to read through the guide, and check out the commonly used builds on pvxwiki.com. The talk pages contain alot of useful information about the reasoning behind skill choices, so if you have the time I would recommend reading those too.
If you have any specific questions, please feel free to post again, however as this question is too vague this thread is closed.