I'm learning Simandl Etude 17 (p. 19 of my book) for my local All State auditions, and I find the G on the E string to the open G crossing to be difficult (and, for that matter, most large difference string crossings). Are there any exercises I can do to help with these big string crossings?

String crossings can be made effortlessly playing German Bow through the use of the minor axis of the wrist, varying the angle of the bow on the string and ever so slight movement of the instrument back and forth as string crossings are needed. If you watch videos of great 'Cellists, you will see how flat the hair is on lower strings, how the hair moves to a clear 45 degree angle on the top string, how the axis of the wrist plays a part in these changes and how their instrument moves ever so slightly to facilitate the entire motion (and even bow changes). This practice transfers extremely well to the Double Bass. The bow rolls from lower to upper strings with more ease, speed and less loss of bow "real estate"with this borrowed technique. In addition, you will find that weight transfers so differently through the elasticity of the stick on a thinner string with more of an angle. Please note that a stiff arm, an inflexible wrist, a static flat hair angle on all strings as well as a complete lack instrument movement are the antithesis of this technique. Please let me know if this has been at all helpful and feel free to ask more questions.
In the spirit of Brotherhood of the Bass,
I remain,
David Ruby

PS-I thought I would say that you can definitely get a great auditon up in the time you have left - the excersises I would recommend are simple - scales with bowing variations and intervals; the best way to become proficient in string crossings is learning the changes string to string and then learning two and three string changes through practicing intervals from unisons through to octaves. Remember that you are your own ultimate mentor - learning through careful observation during practice will solve a great many things. Like the promise of a miracle diet, self improvement is in the hands of the individual.
All the best,
David R