This is a process that requires a great deal of trust, as you must trust that the DM isn't out to get you. In a more nomothetic2 setup, the arbiter of reality is easily observable and, by virtue of being relatively immutable, is judged to be impartial.
However, it was not always this way3. Groups with a more ideographic tradition rely on precedent recorded in the memories of the group and the intentions expressed rather than the specific and strict rules.
What does this mean for you?
First, figure out the story the DM wants to tell, and make sure you're participating in it. For this sort of tradition-bound game, challenging the tradition will not be much fun for anyone involved.
Second, play conservatively with the rules, as they're certain to change. Focus more on narrative components of character that can be mapped to rule systems as they emerge. Don't rely on edge cases in the rules for edges or "power". So long as you have a strongly defined narrative character and the group buys into your character, your concept should be safe from overt tweaking by rules changes. Beyond that, once your character is in the tradition of the game, actions that make narrative sense but aren't supported by the rules will be privileged.
Third, make sure you have a model-of-rules 4 in your head. When something occurs that doesn't match your mental model, discuss it with the group (either out of play, or ask for a pause for discussion) such that everyone can be on the same page with their internal rules predictions.
You may ask... why do we wear these funny little prayer shawls? I'll tell you! They give us a +2 bonus on a skill that isn't in the game any more!
2 See: Clerics, Magic Users, Fighters, and Thieves. To quote myself:
Guba and Lincoln (1994) articulate the nomothetic debate in social sciences where they note that general theories may not fit
specific cases well: “This problem is sometimes described as the
nomothetic/idiographic disjunction. Generalizations, although perhaps statistically meaningful, have no applicability in the
individual case.” While their argumentation is in support of
qualitative research, the theoretical basis of the nomothetic as
“law-making” conflict with the ideographic study of the
individual case maps quite strongly onto the axis of form and the ideas will be used throughout this document. Players seeking the support of rules are far more nomothetic than those seeking mimesis with specific, individual cases of reality or imagination.
3 see: Abused Gamer Syndrome
4 See: ESR's discussion of open source transparency for mental model consequences.