Congrats on your approval!! 
I think you'd have to use the card at least SOME to earn a chance at a GLI (Macy's calls 'em Guideline Increases, for some reason).
If you shop online, check out the Clearance section of their website; that's where I do 95% of my shopping, and I've gotten screaming deals on all kinds of stuff.
In this economy, I'd try to buy something actually needed, not just some random purchase that won't do you any good. Make a purchase and PIF right away. If possible, repeat here and there for the next 3 months, then call and ask if you're eligible for a guideline increase yet.
Your first increases might be a bit small, but Macy's seems to increase in generosity in line with how much you spend and how quickly you pay off, but I also seem to recall reading about customers who've barely used their cards and gotten quite generous GLI's as time passed, unasked for.
Macy's also does auto-increases - I got one at a year.
Hope this helps.
i seems that 99.99% of all users of this forum have thier macys card except me 
i was denied twice in 2007 due to no credit history. i m afraid ll be denied again,
have only one credit card
I got my last increase about 92 days after the previous one, and having not used the card even once (not to mention it being at $0 balance for the duration) during that time. They upped me from $1400 to $2100. Before that I went from $700 to $1400 and had maybe used it twice (spending a total of maybe $100, if that, and PIF-ing) in the 90 days since the increase before that.
When I first got my card - with a $100 limit, in April 2007 - I used it to buy a few kitchen things, then paid in full... then each month I could usually find some random thing to buy while wandering round Macy's (it helped that it was 3 mins walk from my front door, so if I wanted a pair of socks, Macy's was the place to go!), and I'd PIF every time. New wallet - Macy's. Pizza pan? Macy's. New mascara? Macy's. You get the picture. Just buy some socks there if nothing else, at least the card will get some use! I personally love Macy's for the crazily reduced prices they always have on their kitchen stuff (and other things - most things, in fact) - at least, at our local store, if you take something up to the counter and ask them to scan it to check the price, it always seems to be about 60% lower than what's actually on the label. Weird.
If I remember the numbers right, my CLIs took me from $100 - $200 - $400 - $700 - $1400 - $2100 - every 90 days. I'm sure if I used the card more often, I'd have got even better increases than that - still, the day after they gave me this latest increase to $2100, I went out and spent $300 (now PIF'd) - at least I know my card will have had some use when CLI requesting time next (hopefully) rolls around.
When I call to ask for a CLI I say I'm planning a shopping trip to Macy's the following weekend and don't want to risk maxing out my card - I'm sure they don't care what my 'reasons' are, as their computer tells them what CL I'm eligible for regardless of what I say, but it makes me feel less strange about asking for a CLI on a card that's had a zero balance and not been used in who knows how many months!
Congrats on the card - Macy's is a good 'un. First card I got when I moved to the US - they've been good to me. Treat them well and they will do the same for you!