Like finding a sleeping bag that will be perfect for keeping you cuddled up, your choice of Underoos is important, too. If it's chilly out, you want something that will warm your bones and vice versa.
Check out the weather forecast before you head out, paying special attention to nighttime lows. It's all well and good to know how hot it'll get during the day, when you can quickly shed or add a layer and be back to an ideal comfort level. But during the wee hours, it'll be a nuisance if you're waking up repeatedly because you're too cold or too hot. You'll be thrashing around trying to remove a sweatshirt that has you roasting or struggling to find the same item on nights that set you shivering – and no one sharing your tent is going to appreciate the ruckus while they, too, are trying to get a good night's rest. Sleepwear should be dry and free of any sweat, and long underwear plus socks is your best bet.
Another method veteran campers sometimes employ when the thermostat drops is to heat up a hot water bottle before bed and tuck it down into their bags. If you put one near the core of your body (the area of the abdomen) that can help your body's mega heat engine warm the pockets of air in your sleeping bag, and you'll be much warmer for it.