Guidelines for Providers of Ethnically and Culturally Diverse Populations
 
While the APA Guidelines for Providers of Psychological Services to Ethnic, Linguistic, and Culturally Diverse Populations themselves are good reading, I've provided some basic points here.
Educate clients who may not be "therapy savvy."
 
This applies to testing too. I worked with an African American grandmother who didn't care about IQ or achievement testing, or grades really, asserting that she knew her grandson was smart. He failed his then current grade twice though already, was in it third time, and would be repeating if he didn't take a special test, which she has missed scheduling him for twice already.

As a result, he is less likely to graduate, has a lower job potential, and is at risk for more problems as a result. Discussing these sorts of issues with her is likely more helpful than assuming she is negligent or stupid.
Know the Research
 
Be aware of research, such as that of under-use of mental health services by minorities, and the kinds of reasons for this. Many may simply not trust you to "assess" them because they don't trust psychologists, the tests they use, or the uses you will make of the results.
Consider Ethnic and Cultural Experiences
 
Be aware of constructs like "healthy paranoia" in a client who is worried about where his records will go, who will see them, and why you are asking some information.

Consider a client who feels helplessness over information about her that seems to be everywhere but she has no control over who sees it.

What's the difference between a mean and angry upper-SES woman or an assertive and tough lower-SES woman?

Poorer families have fewer resources (both in terms of money, time, and patience) available, often live in dangerous neighborhoods and have limited access to services, and face racism or homophobia or sexism in schools, work settings, and other places…

Likewise, they may have numerous strengths as well, like cousins or relatives to tutor, family babysitters, an uncle who could help a teen get a job, etc…

We will discuss parenting practices (which gets into ideas about "good" parenting and "bad" parenting too) and their effect on test scores.
Consider Family and Cultural ideas about "The Problem"
 
Different cultures and families view problems, mental illness, shame, and responsibility in different ways. This understanding may or may not effect your take on the issues, but could significantly effect how you present them… some creativity is imperative.

Example: The Polish cleaning lady who didn't like insulin
Document Acculturation
 
This can include more typical issues such as how and when the family came to this country, what kind of good or bad life they left behind, and any trauma or stressor that remains, like missing family, family splits, inability to return to the country to see them…

What about the biracial Alabama couple at a high school that cancelled prom rather than have a biracial couple there. This isn't acculturation issues like those that immigrants face, but this case certainly would include documenting issues that have effected their relationship that are a bit more serious than "my mother in law hates me."

Even traditions like having a cotillion at age 16, and going back to Mexico to buy supplies, or to Italy for the wedding dress can have a significant impact on family functioning.