1/ Ice is the most important thing! Good ice cubes will make your cocktail the right temperature and dilution. So make sure you have a lot of ice cubes on hand!
2/ The right tools - shaker, strainer, fine strainer - are all going to make your life a lot easier. A measuring cup (or jigger) is essential to maintain the right proportions.
3/ A workplace: this prevents total chaos and mess. Aim to put aside one small piece of table or counter with all your necessities. These can be such simple things like a cutting board, knife and your other cocktail tools, but it really makes a difference.