Ok, so braid is more expensive line than mono, so why fill a whole spool filled with line that you will never use? Instead, throw on any old mono about halfway up the spool:
Next, tie the two lines together using a uni to uni knot:
http://www.netknots.com/html/double_uni_knot.html Uni to Uni Knot
After tying the knot, apply tape or band aide over the knot so your line doesn't snag on it during casts. Note if you decide to just spool braid on the spool the line will probably slip in place. Another option is to use tape on the spool.
Fill the braid most of the way to the edge of the spool for extra long casts. Note the pic is slightly overfilled, the reason is the braid will lose some diameter as you use it. Leave a slight lip. Be careful during this break in period to keep line tight on the spool to avoid backlashes. Bombs away!
Using this method you will only use about 75 yards of braid at a time. If the line wears out or if your line is too deep on the spool (and thus costing you bigtime casting distance), you can tie the worn end to a tree walk away until you come to the braid mono knot. Cut the braid, then retie the worn end to the mono, retape and spool on backwards. Presto! Fresh line.