The results of any operation are enhance when the patient shows up healthy and ready for surgery. Aspirin and NSAIDs probably should be stopped a few days before surgery but this is not absolutely necessary. Arnica doesn't do anything and neither do vitamins.
Congrats on your decision to go forward with liposuction. Most of us feel that it can offer great benefits and satisfaction even when people are not the "perfect" liposuction candidate (meaning perfect skin and very little excess of fat). Makes sure you keep yourself well hydrated before the procedure and avoid aspirin or herbals that can make you bruise more. There are not any skin creams or topicals that are known to enhance results. But, be prepared to be compliant in wearing your postoperative compression garments. Those help with the shaping and swelling. If you are still weeks or months away from your procedure, continue doing well with your diet and exercise program. Physical fitness will always enhance your shape and ultimate result.