I personally suggest relaxing some initially, my favorite book I read while pregnant the first time was Vicki Iovine's "Girlfriend's Guide to Pregnancy" and she suggests following the "9 months on, 9 months off rule." (i.e. it took your body 9 months to put that weight on, give it 9 months to get rid of it as well!) I adapted that into my own life as "Focus on the baby when they're first born and don't stress yourself out about your weight. ENJOY the product of your labor!"
Around 9 month mark, the baby is more stable, sleeping through the night, eating some solid foods, your milk supply isn't inherently a crucial problem anymore albeit you should still pay some attention to it, ANYWAYS, THEN you can start berating yourself about your weight gain/loss issues.
Give yourself a break at first though! Relax, sleep when you get the chance to sleep, and focus on yourself and enjoy your new baby! Focus on eating HEALTHY vs losing weight
A guy I know from another forum who is becoming a personal trainer says its more important to think to yourself: "What can I
add to my diet this week to be healthier" vs "What should I deprive myself of this week to lose weight" <---this concept is especially important for new moms I think.