When you are purchasing your items in the check out line, take a moment to speak with the employee or clerk helping you.
Ask them how they are doing, or if they have plans for the weekend. A little bit of pleasant conversation can go a long way. Also be sure to thank them for checking you out or loading your bags as you leave.
Everyone wants and needs to be appreciated. The act of starting a conversation with someone who is helping you is a small way to show your appreciation for their work.
If this is a store you often frequent, then you could potentially build a friendship with this clerk. Make an effort to remember previous conversations and pick up where you left off. Around the holidays you could even bring them a card or treat of some sort.
If you aren't feeling up to conversation, then simply smile and make eye contact. Thank them when you leave. You can let them know you appreciate them without saying much!
A list of unique conversation starters to help you break the ice.