My $.2
I have worked in the service industry, both in tipped and non-tipped positions, and still have many friends who work in such positions.
When giving equal funds, something that shows the tip is 'personal' is always, always very much appreciated. Whether it be handed with a card with a personal note, or money made 'fun' in some way (I used to have a regular customer who did origami and folded the bills into interesting shapes) it's a nice touch. It says "Not only did I like your service, but I liked dealing with you as a person and I really thought about /you/ as something other than just a machine that had to be paid."
However, 'interesting' tips do not make up for a difference in cash. No, two $2 bills are not going to be liked as much as one $5.
The best way you can make your tip matter more than the dollar amount? Give a 'tip' to their manager, and let them know that you got exceptional service. This is true for anyone who works in the service industry. Normally managers only hear complaints, if someone is giving good service, it goes unnoticed. Letting the manager know that they have a good worker means that that worker gets put up for more promotions, can receive bonusses, etc. etc. And believe it or not, a lot of people who work in the service industry really like the aspect that they're helping people, and to know that their hard work was noticed can really make their day.