I prefer to route my dynamo wires internally, though fishing cables through the frame can be a test of one's patience. Typically, I start with an old length of brake or shifter cable to find the path, and then use that to pull my cable through.

On some frames the cable can be routed directly through the opening where the down tube and chain stays enters the bottom bracket. I have done this on more than one bike. On others where I don't have the means of removing the bottom bracket, I have routed the cable out the bottom of the down tube and under the bottom bracket as described by niknak. I also second the use of connectors where it may be desirable to separate parts (for me, that is where the taillight wire on the rack meets the wire coming through the frame).

Whether I route the wire through the fork really depends on the shape and material of the fork.

In any event, I make sure I've got my plan figured out before drilling any holes. Typically, I use rubber grommets where I drill holes. I still file down the sharp edges of any holes, though to keep from cutting up the grommets.