Learn the basic theories, techniques, and classic ratios. They will help lend you the structure needed to later get creative. A great first step is to learn when a cocktail is meant to be stirred and when a cocktail is meant to be shaken. Aromatic-style cocktails, also referred to as 'boozy' or 'spirit-forward' contain only alcoholic ingredients and sugars; think Old Fashioned, Manhattan, Martini and Negroni. Stirring allows for dilution and chilling and creates a textured and silky final product. Sours, flips, and everything in between require shaking for the added aeration and homogenization. Shaking creates a frothier, often lighter texture. Shake whenever the cocktail incorporates citrus, cream, eggs and other fruits.
Ice is the most important ingredient in your drink. Add your ice last. I know, I know, this is quite counterintuitive. Building your cocktail first allows you to have more control over the dilution and how quickly your drink gets cold. Check out my blog post on ice to learn more.
Use real ingredients, not pre-bottled juice. Everything you do matters and your cocktail is only as good as your worst ingredient. Grab a lime. Squeeze it. Use quality spirits... this doesn't mean expensive! Stay away from pre-made syrups full of ingredients that won't add any value to your cocktail.
Work on balance before you introduce new spirits or new flavors. Perfect the Daquiri, perfect the Old Fashioned. Understand dilution and how it affects different cocktails. Shake a cocktail, shake 100. Recognize the change in sound as the ice cracks and your cocktail changes into its final form. Stir a Manhattan with a 1:1 ratio. Stir a Manhattan with a 2:1 ratio. Do them at the same time, taste the difference. Find out how YOU like YOUR Martini.
Measure EVERYTHING. Remember that you aren't making a huge pot of chili, you're making a 3-5 oz beverage. Every .25 oz you change will make a huge difference.
Most importantly, if you're in charge of making drinks for the night (hey, bartender) don't judge people for what they're drinking.