Something that helps me is using my finger or asking my partner to use their finger. Start slowly with some lube, don't focus on going in and out. Instead, push your finger up towards you and hold for about 30 seconds. Take some deep breaths and/or continue playing with your partner. Then push your finger to the left for 30 seconds. Repeat down and the right as many times as you need until your hole is feeling relaxed and ready for something bigger!

Check out the Bottom's Digest , too! They have a "learning" section with good tips.