Summarize this article:

Most people won't pull out a set of measuring cups or a food scale at a nice restaurant.  To keep you on track with portion sizes, have a guide on hand like this one from WebMD  Keep a copy of this guide in your wallet or purse.  Pull it out once your meal arrives and compare what you're served with and the appropriate portion size listed. Divide your plate by the "correct portion" size and "leftovers."  Ask to box the leftovers immediately so they're no longer a temptation. Do a little research before you head out to eat.  Knowing what options are available prior to ordering helps you stay away from temptations and stick to your smaller portion size.  Check out the menu and nutrition information online if available.  Choose 1-2 items that seem good and will fit into your small portion. Call ahead and see if they're able to half or smaller portions.  It can help you prepare for whether you will need to take home leftovers. Learning to leave a little bit of leftovers on your plate is a good habit to get into.  You can almost guarantee that restaurant portions are much larger than recommended.  Plan on having leftovers.  When you eat out, ask the waiter to box a quarter—or even half—of your meal before you get it. You'll have the perfect leftover portion for the next day. Ask your server if they have half or petite portions if it's difficult for you to leave items on your plate. Split an entrée with a friend or family member.  This is another good option if you have a hard time not finishing the whole portion.  If you split it, it's automatically portion controlled! This is a great way to manage portions.  Starters are usually smaller (and cheaper!) than entrées.  There's also a low risk of finishing "more than you should" which can happen with larger entrée sized meals.  Try tapas too!  Some restaurants serve tapas which are very small portions of dishes.  You can order 1-2 small tapas and overall still maintain a small portion size.  It's great for those who like to try a variety of dishes. If you don't order an appetizer, try order a child's size portion.  Again, these are much smaller than an adult entrée and make portion control a lot easier.  Beware of "kid foods" - sometimes portions are smaller but the choices might not be very healthy.
Print or buy a portion size guide. Choose your meal before you go. Leave food on your plate. Order an appetizer for your main course.