Dear Troll
Getting someone who has faith in your abilities to give you a chance is always the hard bit - I was very lucky in this regard after bobbing up and down between digger and assistant supervisor for years. Initially getting supervisory experience on (paid) training digs i.e. to teach, either in the UK or abroad was one route which helped. I could then truthfully say I had previously supervised staff even though the staff themselves were volunteers. The pay wasn't great, probably once you worked out the free accom about the same as a normal diggers wage but the experience was amazing and proved very useful. Getting in touch with your old uni (if you went) or a local uni and telling them about your commercial skills could help here. If nothing else they might like a careers talk or a 'from the coal face' discussion at their student seminar series or for archsoc. Volunteer societies won't pay (unless you can put a HLF bid together) but if you don't mind doing a few weekends training them in tape survey skills, section drawing or something of that ilk again you can truthfully say that you have supervised groups.
Getting pieces of paper can be of some use but the two I can say have definitely bumped me up out of the diggers role were driving license (and own car) and first aid certificate. This may be an old chestnut but I can only remember 2 supervisors in a 10 year fieldwork career who didn't drive. One was at a uni unit and always paired with an assistant supervisor who did (very understanding of that unit) and the other was willing to ride his bike 20 miles there and back for a watching brief in torential rain. And he still got whinged at for being slow. Being the person who's early on site and has their own wrench (?1 ASDA, literally changed my career) to open the heras fencing when the supervisor forgets his. Being the person who takes the EDM back and charges it and who has noticed we're low on film and ordered it. Having the first aid cert (free from the job centre) so when the boss can't work weekends or late the site can still open cos there is a qualified bod to take care of that requirement. I'm a competant hardworking digger not a super-duper fantastic one with loads of finds knowledge but I'm so fecking organised you would not believe and that sort of thing gets noticed. My context sheets were mint (and referenced other peoples for an area view not just what was happening in my own slot). And after years of turning up just in the nick of time i finally realised that 'on time' in a supervisor's or PO's mind is actually 10 minutes early. Really.
As you've been around for a while, if you have done all these things (you probably have but I'm trying to make this applicable to anyone reading) but are still not breaking through how about
MoRPHE distance learning free EH training
http://www.english-heritage.org.uk/profe...ng-morphe/
Trying to book on the free CPD courses run advertised on HELM
http://www.helm.org.uk/server/show/ConWebDoc.11473
Evening courses in CAD down the tech (less useful than 5 years ago though as the tide seems to be moving on on this one)
Anything to do with GIS at all ever, even if its just downloading some freeware version and mucking about with it
Finally I have a friend who is a better digger than me. She was an assistant supervisor before me and she stayed an assistant supervisor while I moved on and up to supervisor and project officer (good times) and eventually sideways out of fieldwork altogether. She had a partner who worked in a certain region and so she stayed with the same unit even though her efforts were unappreciated and her colleagues didn't rate her. She kept her head down, never caused trouble and worked like a dog with a cheerful and uncomplaining attitude. While there's something to be said for sticking around and not jumping ship too often before then end of the project at the end of the day these people didn't like her and were never ever going to promote her. Sometimes it really isn't you. So, without encouraging paranoia, have a think about whether the local bods you have been working on and off for for years are actually ever going to see you in a different light. Have a chat with them and see if they are willing to give you a go. And if not then move on to somewhere that can see your potential. Because eventually she got so discouraged that she quit and now works in a supermarket and that is a crying shame. I worked for the same unit for one year and when they promoted the male best mate of the illustration officer (2 years my junior with correspondingly less experience, plus a degree from the same uni 2 grades below mine) above me after promising me the job you couldn't see my heels for dust!
Good luck with this troll, its a mission to change people's perceptions of you but it can be done. I don't think anyone from my current role would recognise the bed hopping drunken pipeline shovelbum of ten years ago but I am still me. I loved that time of my life and I don't regret an instant of it but I have grown up and that has its own rewards. The people who didn't promote me (or my hardworking friend) now work for me on occasion and while I never make an issue of it thats a damn good feeling to have ;-)
one girl went to dig, went to dig a meadow...