plpgsql , dynamic queries

Hi,
I am having problems in forming a dynamic query that can be used in
Execute statements.

Problem:
I have database fields that are either INT or VARCHAR. Both can have NULL.

If I use variables that hold the db field's values ... how do I need to
quote them to pass them to the query string.

example:

_var1 := NULL;
_var2 := ''NULL'';

or
queryStr := ''WHERE name NOTNULL";
queryStr := ''WHERE name ='' || _var2/1

_var3 := ''somename'';
quereyStr := ''SET name='' || _var3;

I tried combinations but ending up with using 4 or 6 single quotes. Is
there an easy way to form such query strings ?

Any help would be appreciated.

Thanks