first of all, try to find which output is fastest on your system. Start dosbox with different options (surface, overlay, opengl...) and check processor usage - then use the one with the lowest usage. Also, use dynamic core whenever possible. If the game doesn't start with dynamic core and uses a dos extender you might try to change it with dos32a or alternatevely you can find a recent compiled cvs version with coreswitch patch which will enable you to switch to dynamic core once the game was started with normal core...

Or start with pressing ctrl+F12, when you did that a few times and your cpu usage is 100%, you might try to press ctr+F8 to increase frame skip.

If that's not enough, do what's said in the post above mine.