When you find a translator you like, you can add him/her to your translator list. By doing this, you will be able to send a request to him/her.
A list can be created from the left sidebar marked 'Translators' on your requests page, or by clicking "Add to list" button on the activity section of a completed request. You can create several different lists, and include the same translators in multiple lists.
As for sending a request to your favorite translators, on the 'Checkout' page of a request, select 'Premium Request Option' and add your favorite translators to send your translation request.
What is a premium request and a translator list?