The most prudent approach is to make the leverage uncallable.
If you own 1000 shares of a stock, and want to control, for example, 1100 shares, you could sell 100 shares and buy 2 option contracts of the longest expiration and lowest strike price possible from the proceeds, for even money. That way you don't have margin loan and have zero risk of a margin call.
The implied interest rate for BRKB deep ITM LEAPs currently is around 6%, not too different from any other cost of leverage. (Probably a bit high for the expected return of Berkshire at the moment).
Jim (mungofitch) on the old Fool boards did a lot of great work on this.
Eplanation of implied interest rate:
http://www.datahelper.com/mi/search.phtml?nofool=youBet&mid=34391366
http://www.datahelper.com/mi/search.phtml?nofool=youBet&mid=31996045
One nuance: During a steep drop in stock price, you might be able to lock in some gained time value by rolling the strike price further down, as explained here:
http://www.datahelper.com/mi/search.phtml?nofool=youBet&mid=31899968
Another nuance: If the stock goes absolutely nowhere, or down, or up less than 6% p.a., you would have to roll those options forward (a few more years out), in order to maintain the leverage. This may require fresh funds, suitable strike prices may not always be available, or implied interest rates could be even higher.