They're sorted alphabetically so one idea is to add a prefix to your custom fields.
If you really want, you can manipulate this shadow tiddler: $:/core/ui/EditTemplate/fields by inserting some sort operator in the listwidgets filter. Not really recommended.
You question is related to the ordering of editview templates so I mistook it for that first, to which I was about to reply the following:
>Yes, you use the list-before and list-after fields to state what other field template it should appear before or after.
>If you want it very first or very last, you can simply use an empty list-before or list-after field.
...The thing is that all the things you see in editmode (as well as in viewmode) are fields - for example the text field or the title field. The order of these are controlled as I described. So you *can* create a separate editviewtemplate that contains your field and really control where it is positioned by the described method. To create an editviewtemplate you simply create a tiddler and tag it $:/tags/EditViewTemplate
Well, how about that for an over the top answer .
<:-)