(1) Be able to write. The better you are at writing, the easier your graduate school experience will be, especially in the first few years. In all of your classes you will be asked to write: essays, papers, laboratory write ups, analyses, reviews, etc. Writing is a key part of graduate school. In fact, its basically the product of archaeology. At our core, we are writers who happen to rummage around in dirt, speak conceptually about rocks and other kinds of inanimate things. The better you are at writing, the better an archaeologist you will be. Before you start graduate school, do some writing. Start a blog and keep it current. Turn a paper you wrote as an undergraduate into something for publication - whether that is a newsletter, local publication, regional venue or greater. Take a class in writing at your local community college. Read Strunk and White's Elements of Style. Then read it again. Then read On Writing Well, by William Zinsser. Then practice some more. Make your writing logically sound. Clear. Sharp. Readable.
(2) Learn how to use Microsoft Excel or any equivalent spreadsheet program (whether that be OpenOffice, Gnumeric, Numbers or some other variant). Given that contemporary students are often touted as being terrifically "computer savvy" with all of that YouFace, FaceTube and Twitface stuff, I am incredibly appalled that many entering archaeology students have never used a spreadsheet. I've seen students, for example, enter numbers into a spreadsheet as instructed and then do calculations on a calculator (e.g., summing the values) and then entering the results back into the spreadsheet. Really. Given that much of what we do in archaeology in addition to writing is counting things, have advanced knowledge of Excel will go along way to making your assignments be about the problems posed and not the mechanics of dealing with numbers. Trust me on this one. Learn how to use a spreadsheet, create headers, use autofilter, use functions, create formulas, create graphs, do basic statistics, etc. Equally important: learn how to edit the graphs and tables produced by Excel to make them acceptable. Don't rely on the defaults. Learn to identify "chartjunk" and maximize the information to ink ratios for your graphics.
(5) Learn something about GIS. In graduate school, you will definitely have opportunities to learn something about spatial systems. However, you should gain a background in the basics of geographic systems before you start graduate school. GIS is really just a different kind of computer application - like Word or Excel - that allows you to manipulate concepts in space. No archaeologist worth her/his salt should lack an understanding of GIS. And that means you. The more you know about GIS - like using Word or Excel, the easier it will be to focus on archaeological analyses that deal with space rather than the mechanics of the various program that you are going to use to study space. Learn ESRI's ArcGIS or the open source Quantum GIS or one of many other geographic information systems.
Well, that is a start that should keep you busy for the summer prior to starting a graduate program in archaeology. As I recall other critical skills, I'll update this list. If you have any ideas, please let me know.