Go during the off -season. Myrtle Beach's off season is usually any weekend between Labor Day and Memorial Day. It is a little cooler so bring light jackets, but 90% of the fun in Myrtle Beach isn't the water. That being said, you want to avoid special weeks in Myrtle Beach like the two bike weeks (one for Harleys, the other for speed bikes) and the Shagger's convention. Unless you have a special affiliation with those hobbies, do not go to Myrtle Beach during those times. The most expensive part of Myrtle Beach are the hotels and prices are sky high during those times. Everyone will make you a deal during the off season.
Pick the right hotel for your family. Why spend copious amounts of money for a souped-up hotel room that you will only see early in the morning when you leave and late at night when you get back? Or why not spend a few extra dollars for the hotel that comes with a mini-water park and great restaurants inside? Picking the right room for your type of trip will save you lots of cash. If you have a family with small toddlers who can't take a ton of walking, get the hotels with the water parks. If you are going to explore Myrtle Beach from 8am to 2am, head towards the edges of town.
Fill up on gas in the right towns and buy a Myrtle Beach road map. If you are coming from Florida, fill up in gas just as you get to Georgetown, South Carolina. It's the last stop before Myrtle Beach, where you can spend a lot of gas cruising on 17. If you are coming from the North, Florence has the least expensive gas. The roads aren't complicated in Myrtle Beach but remembering which exit to turn off of can be. If you miss your turn you can expect a good 20 minutes to make a U-Turn.
Focus on Broadway at the Beach or Barefoot Landing. There are a multitude of activities to do for all family members at those two locations. By parking in those areas and walking around you can save gas money from driving up and down the strip just hoping you can get into the right lane for the other overpriced restaurants and attractions. Broadway at the Beach has Ripley's Aquarium, Planet Hollywood, a small kid's carnival and dozens of great shops. Barefoot Landing hosts the House of Blues and the Alabama Show Theatre.
Eat one meal in every day. Whether it's breakfast, lunch or dinner, eat at least one meal in. Most of the family hotels have small kitchen units that are more available during the off season. Even if you don't eat in, take a picnic lunch.