this isn't exactly a new tip, but if you don't know, you can select a random id from a table using the following syntax:

select top 1
  f
from
  t
order by
  newid()

so... if you need to perform random performance tests, you can use the above to retrieve a random id and then test against it. here's an example:

declare @v varchar(20)

select top 1
  @v = cityname
from
  zipcode
order by
  newid()

select
  *
from
  zipcode
where
  cityname = @v

  

very simple, yes. but effective for quick testing. =)