Hi wishful,
Like Mama J said, there's no way to make you keep the swelling longer, some have the luck of it sticking around all day, while others don't.
Some things you might try to help with your swelling would be to make sure you're intaking plenty of water, the more water the better, it will keep your skin nice and hydrated and plump, which helps it swell and helps it stick around a bit longer.
Another thing you might try would be drinking protein shakes, they may or may not help, depends on the person.
Make sure you're giving them a good massage before you noogle, it usually helps me get some extra swelling from noogling that lasts a little longer than if I don't.
Taking a hot shower or soaking in a hot bath that covers the breasts seems to help with the swelling, I think a hot soak afterward works a bit better than a hot shower, but if you do take a hot shower just let the hot water run down over your breasts, try not to let any hard stream of water hit them, I've noticed sometimes if the water strikes them directly it doesn't help with the swelling, where if it just trickles down over them it does.
Another thing that works for some is to wear a good supportive well fitting bra that's tight but not too tight, with straps that run down the sides more near the armpits rather than down the middle towards the nipples, the ones with the side straps help to support and shape them better. There's also some breast shapers on Ebay that are supposed to help take the fat from your under arms, upper arms, and surrounding area and pretty much relocate it to the breasts. It's worn with a bra, it draws in all the fat from around the breasts into the bra with the breasts and eventually after you wear it every day for a while it's supposed to make the effects permanent. It may help hold the swelling and make it stick around a bit longer, not sure though because I haven't been able to get one to try it yet. Here's a few links so you can see what I'm talking about...
http://cgi.ebay.com/5007L-Breast-Shaper-PUSH-UP-2-CUP-SIZE-UP-Lagre-/170471453701?pt=AU_Womens_Clothing_2&hash=item27b0e3f405
http://cgi.ebay.com/NEW-BRA-BODY-SHAPER-CHIC-BREAST-SUPPORT-32-38-/150468945306?pt=US_CSA_WC_Intimates&hash=item2308a5e59a
http://cgi.ebay.com/2-Cup-Bigger-Push-Up-BREAST-SUPPORT-BODY-SHAPER-BLACK-M-/160489094521?pt=US_CSA_WC_Intimates&hash=item255de53d79
http://cgi.ebay.com/breast-UNDERBUST-bra-BACK-SHAPER-chic-SUPPORTER-XL-NEW-/360305616609?pt=US_CSA_WC_Hosiery_Socks&hash=item53e3e366e1
http://cgi.ebay.com/Magic-Bra-shaper-Bust-Lifter-Breast-lift-Sexy-Shaping-M-/150491522072?pt=US_CSA_WC_Intimates&hash=item2309fe6418
Oh, and before I forget your health can play a crucial role in how much swelling you get and how long it sticks around. And if you take any anti-inflammatory drugs for anything it can cut down on the amount of swelling you get and how long it lasts. Also, while some people have had awesome results taking EPO and/or Spirulina, I took them both for a few months and noticed that I did not get as much swelling while I was taking them, and sometimes it was gone in a matter of a few minutes to an hour or two. Before I started taking them I was getting some awesome swelling, now that I've stopped taking them it's getting a little better, but nowhere near as good as it was before, guess it takes time to get their effects out of the system and get back to where I was. So unless you have to for health reasons I'd avoid anything that reduces swelling for a while and see if that might help.
Hope these ideas give you something that will work for you.
;D