Hi PotterGrl and All,
As JBaymore mentioned, plastic is king. I start with a beginning till of $50-$100 (depending on the size of the show) in 1s, 5s, and 10s in a bank envelope in my front pocket... good luck with taking that. But at least 1/2 of my transactions are completed with credit cards. Local farmers' market to large weekend shows... plastic always accounts for at least half.
An option that hasn't been mentioned is ProPay. I used a Square reader to accept credit cards for a few years, but recently registed for some weekend shows in rural areas without data/wireless service. Square is great if you always have a connection, but if you're off the grid you're SOL unless you keep a knuckle buster with you to process those separately through a processising service... even though you can physically punch in a card's information into Square and process payment it is technically in violation of the Square contract. It was not something I was willing to gamble with. I've also been at shows where the signal is spotty, and I've had to trod around with my phone in the air hoping to catch a wave of 3G for a transaction to go through... awkward/embarrassing for me, annoying for the customer. MarkC also mentioned being in situations where the Square server gets overloaded.
So I recently switched over to ProPay's JAK Reader ($20 hardware), which just like Square, uses an app supported by Android or iOS. I use an iPhone5, but any Driod phone or tablet or iPad will work as well. The real advantage to ProPay is that you can run it in "Suspended Mode" when you're in an area without service. This stores your swipes to process later. In this mode, everything looks the same: card get's swiped, customer signs and provides and email for receipt. Once you have data or wireless service you open the app and process the stored transactions. The risk of a declined card is still present, but no more so than with a knucklebuster and you only have one (modern, clean, quick) system to navigate. When operating in an area that has spotty service I've found it helpful to run my phone in Airplane Mode so the card reader doesn't get confused by the in-and-out signal.
The annual cost of the service is $50 and the percentage (2.7% for MC, Visa and Disco - 3.4% for AMEX) per swipe is comprable to Square. Transfers to your checking account cost $0.35 whether you are sending the funds from one transaction or a whole weekend's (or year's) worth... I do this once at the end of a show. There is no flat rate "per-swipe" cost in addtion to the percentage.
Cash is a must. Security of your cash is as important as the cash itself. But not having the option to take a credit card is a recipe for lost sales. If you're only showing/vending in areas with consistent service, Square is great. But if you're like me and sometimes show in more rural settings (or don't want to rely on 3G/wireless service at every show) ProPay is definitely worth considering.