You can't, essentially. You can't dictate to people what form their generosity should take. You can, however, let people know verbally, if they contact you and say, Oh, I have no idea what Prudence and Cuthbert would like, do you have any ideas?
And really, one would hope the gifts would be something Prudence and Cuthbert would be able to look at in years to come and remember the giver with fondness.
If you let family know what the happy couple's plans are as regards moving to and from Hawaii perhaps they will take this into account when choosing gifts. But if some wish to pass on family heirlooms, for instance, that's their prerogative.

I trust that you are not hosting the shower, which would be a huge no-no,though it is fine for moms to assist a hostess as long as your name doesn't appear on the invitation. Asking for gift cards tends to negate the reason for having a shower, to "shower" the bride with items she can use in her new home. I agree with Colleen that you can't dictate what guests should give, but you can remind guests that gifts will need to be shipped to Hawaii, so the couple would prefer lightweight items. Having a lingerie shower is one way to solve the problem. If the couple really don't need gifts, you might consider having a recipe shower or something similar. You might even forgo gifts and have a tea or luncheon for the bride's friends and family rather than a traditional shower.

I agree with the other posters (and I'm touched by the use of "Cuthbert"! :-) )
MOTB14, search this forum for strings on the idea of gift card showers (mostly negative, I'm sorry to say) and on the problem of showers for brides who have to travel and/or move. People have written a lot about these topics.

In a nutshell, many people dislike "gift card showers" because they seem too close to a "cash shower" and because they are very boring to attend. Avoiding the boredom by skipping opening the gifts in the guests' presence is even worse -- then it's just an admission charge. (I realize that many people consider watching someone open gifts to be boring, too! But that's what a shower is.)

At my daughter-in-law's shower, in her home town, most of the gifts came from her registries at 2 big stores with branches in most cities. Both stores were very happy to help us: her mom and I simply took the gifts back to the 2 stores in that city, and the stores shipped them -- free -- to the city where the couple lived. (Our original idea had been to return the items for store credit and then for the couple to use the credit to buy the identical items at the branches in their city, but the stores for some reason preferred to ship.)

In your situation, I wouldn't worry about it. Anything that is too hard to ship and that they don't need to have immediately can stay at your home or someplace until they move back from Hawaii. My kids still have plenty of stuff at my house (including a couple of wedding presents).

But the bottom line is, don't over think this. Most people have a few little problems moving stuff from one place to another, and that's just the way it is. Don't make your guests be the ones to solve it. (One poster even asked if she could ask the guests to pay for shipping!)
http://ths.gardenweb.com/forums/load/wedding/msg0309412323245.html (this one even includes a post involving a move to Hawaii)
http://ths.gardenweb.com/forums/load/wedding/msg120914427306.html
http://ths.gardenweb.com/forums/load/wedding/msg0500440511886.html

Our daughter married a young man from across the country. His family is an old, established family in the community, and so they received many gifts from townspeople. The practice there apparently was to call the groom's family to pick up the gifts at the few shops they have in town. That created a huge expense for his parents to ship things across the country. Oftentimes, the cost of postage was as much as the price of the gift itself.

However, your daughter will have to pay to ship the gifts from New York to either your home for storage or Hawaii. Then if he is finished with his stint in the service, they will pay to ship things back again. That will be costly. I think Sweet Pea has some great suggestions. I also suggest that your daughter see if she can register for lingerie if she decides to go that route to help people know what size and style she would like. I would not know where to begin to make a lingerie purchase other than Hankie Pankie lingerie, which I bought my daughter-in-law for her wedding day. Yummm!

It's my understanding that the military will pay for a final move after someone leaves the service, if they get an honorable discharge. So while the wedding gifts might have to be shipped at your daughter's expense *to* Hawaii, the military will pay for them to be moved back to NY once her husband's tour of duty ends.

There's also the fact that many young couples are living in small houses with little storage room and can't fit all their wedding gifts in their quarters. I know several sets of parents who are storing some of the larger gifts in their own homes until the newly weds can afford to move to a larger place. Could you or another close relative store any of the gifts for a time? It's not ideal, but it is a possible solution for large, heavy or bulky things.

I'd suggest not registering for fragile things, if you are shipping them. Avoid the fine china and crystal, and concentrate on things like sheets and nice towels and blankets. Not hard to ship and won't get broken in transit. Small kitchen gadgets would be another category to consider. Or a recipe shower where each guest brings a copy of a favorite recipe, and either a kitchen gadget or spice or flavoring needed to make it. There are plenty of shower themes that would allow guests to bring a small, easily packed and shipped gift.