Configure Developer Dashboard in Sharepoint 2010

How to enable/disable it on the specific site?
Method 1: [STSADM]
'On' Mode :
STSADM –o setproperty –pn developer-dashboard –pv On

'Off' Mode:
STSADM –o setproperty –pn developer-dashboard –pv Off

'OnDemand' Mode:
STSADM –o setproperty –pn developer-dashboard –pv OnDemand

Suppose you'd like to only display the developer dashboard if one or more acceptable values are exceeded, there's a way to do that too by running:
STSADM –o setproperty –pn developer-dashboard –pv expensiveoperationsonly

Once you executed the command the output is "Operation executed successfully."

Method 2: Windows Powershell script:
You need to open the windows power shell prompt and copy the below script and hit enter.
$DevDashboardSettings = [Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings;
$DevDashboardSettings.DisplayLevel = 'OnDemand';
$DevDashboardSettings.RequiredPermissions = 'EmptyMask';
$DevDashboardSettings.TraceEnabled = $true;
$DevDashboardsettings.Update();

ref:

http://praveenbattula.blogspot.com/2010/05/developer-dashboard-in-sharepoint-2010.html