Even a simple visit to their profile will let your match know that you have shown interest in them. However, if you would like to take it one step further, why not try out the following?

• Send a personalised message - Introduce yourself to your match and tell them a little more about yourself. You can do so via their profile, or via your Matches list.
  
  
• Send a Smile - You can send one as a greeting or simply to express your interest. You'll find this option directly on their profile or via your Matches list.
  
  
• Like a profile entry or photo - Like a photo or profile entry by clicking/tapping on the thumbs-up button.
  
  
• Send a photo request - This will add you to their Matches list and they will be prompted to upload a photo.

When you perform one of these actions, your match will receive a notification and see the interaction in their Messages.

These actions are only visible to the sender and the recipient, and cannot be reversed. 

If you delete a match, or a match deletes you or their profile, your respective interactions will no longer be visible.