Develop your breathing. Since playing trombone my breathing has developed 3 fold (has same range as baritone sax). Take in massive abdominal breaths so that you can blow more slowly but also consistently.
My contribution, anyway - never played a Baritone.
Play long tones. Lots of them, probably even more than that. It comes down to the control you can exercise over your instrument. Once you have it, you'll be surprised at how easily the low notes on baritone speak.
Control of your breathing is one of the factors in producing piano/pianisssimo notes in the low notes of the bottom register of the bigger saxes. Other factors include reed selection - a softer reed will help get the low notes but sometimes at the cost of having the really high ones more difficult to attain. Too tight an Embouchure doesnt help either. Another factor is having an instument that is leak free and regulated well. Lastly and most importantly practice practice practice.
I do some exercises that I found on Tim Price website: "Tim's long tone warm-up" and "Tim's low register sax workouts"
Overtone exercises are always good.
I use to practise longtone and overtone exercises every time I'm playing. As warm-up, in the middle or last thing before I clean my sax. Try to achieve a "full tone" and good sonority. Listen to every note and make it to sing as good as possible. You should also try to do exercises that help you to get an even tone over the whole register.
I found Kelly Bucheger's "The Daily Grind" - the section on Long Tones really very useful for low tones - enabling much more consistency and ability to hit the low notes when and how you want them. You can find it here http://www-cs.canisius.edu/~bucheger/SaxPages1.html .