I feel your pain--this is so easy to do!
After casting on but *before* joining your round, lay your needle flat on the table and flatten out all the cast on stitches. Make sure they all lie toward the center of the loop of your needle. Now pick up the needle and join the round, being careful not to twist either end before they are joined. (After joining you can check again, too, if you like, before getting very far in.)