Unique UUID per transaction/sample

Hi All,
I'm trying to get unique UUID per transaction and use it at multiple places
in my test plan.

My test plan is as follows:

Test Plan
|---- Thread group
|---- User Defined Variable
| myuuid = ${__UUID}

|---- HTTP Header Manager
| (use ${myuuid}

|---- HTTP Request Defaults

|---- HTTP Request 1
| (use ${myuuid})

|---- HTTP Request 2
| (use ${myuuid})

|---- Some Listener
The problem here is that it is generating 1 UUID per execution of test plan
rather than per sample.
I've refered JMeter manual here
<http://jmeter.apache.org/usermanual/functions.html#__UUID>, but of not
much help.

How do I overcome this?

Regards,
Aditya