This is a question i have been pondering for a while, say for a catering event or a busy day in a restraunt, does this sauce have to be made to order?
Are there any tricks to make it hold longer?
Beurre blanc has an extremely short life. On the other hand, beurre Nantais -- pretty much the same thing but with a little cream -- is quite robust.
IIRC I've already posted the recipe on Chef Talk along with a substantial amount of discussion. In any case, beurre Nantais is all over the web.
We make beurre blanc for groups and I've found that it can be kept stable over the course of the day without additional stabilizers or cream as long as you keep it just above the melting temperature of the butter (oddly enough the shelf above the deep fryer at the restaurant hits that perfect temperature) and you stir it every hour or so. We use a reduction of white wine and white wine vinegar, bay, shallots, peppercorns and a bit of lemon juice to which the butter is emulsified into it. Season and you have a sauce that is super stable as long as you know how to take care of it.
I have never kept beurre blanc over the course of a day but it is definately ok for a 3 or 4 hour service. I have kept it in a bain marie many times although you do need to keep an eye on it as blueicus says and you need to whisk it every now and then. If it breaks whisk in some reduced heavy cream (nantais).
Blueicus has the right idea. It should hold at the right temperature. Only lasts for one service though. If, for a catering event, where there is transportation necessary, or a need to make a large batch, then I'd reduce the wine with cream to be sure it holds. If you serve Blanc on a banquet item and put it in a high temp hot box for holding prior to service, the cream is insurance that it won't be broken when it hits the guest.
Just my two cents.