It is a pleasant surprise to start using CRM 2011, if you are coming from from MS CRM 4.0 where you have to write a lot of code to get the filtered lookup working. Now CRM 2011 provides filtered lookups out of box. I was really excited after using them so though of sharing them with all of you. Follow the steps below to get filtering on the lookup
We will start with an example where we suppose that we have to filter contact lookup for a particular account (If you are familiar with MS CRM then you will be knowing about the famous parent Customer(Account)
lookup on the contact form), we are going to filter all the Primary contact lookup to only fetch those contacts who have parent account set to a particular account
step1-Go to the "Account" form, open the form customization (you can find this in the top right corner of the account form). Once the form customization is open find the Primary contact lookup field and open it by double clicking.
step 2-Go to the "Related record" filtering section of the field property (the "Primary contact lookup dialogue you just opened). Here you will find a drop down which has the following label "Show only related records. Select Contact(Parent Customer (Accounts)) and select current record.
step 3- If you like you can also select different view from the view selector property once done click ok on the dialogs.
After this save the form customization and and publish the customization. Now if you reopen the account form you will see that the "Primary contact" has been filtered and now only shows the contact who work for this account.
Hope this helps