To get flowers, hang around the bread and reagent vendors in TB - wear cologne since they're female.
To get pies, hang around the AV chick next to the bank in Org - again, cologne since she's female.
To get poems, hang around the cockroach vendor on bottom level of UC next to the bank - he's a guy so you've gotta wear perfume.
Be sure to bring some alcohol when you're camping the roach vendor as he's also the one that you have to kiss and throw a handful of petals at - if you have a Brewfest pony keg then drop that and be sure to shout to the city to let peeps know its up. If not and you're doing this tonight, then buy a stack of that moonglow booze from the Lunarfest merchant. You have to be drunk when you throw the flower petals.
The achievement to share the romantic picnic has to be done in DALARAN (per quest description, but that didn't stop peeps from spazzing all day that they weren't getting the achievement). You do not have to own the picnic basket - if someone is nice enough to be sitting around with one up then use theirs - after sitting there about 10 seconds you'll get credit.
Edit: You can't get the Bouquet of Roses at level 80 by going in and smacking an old world boss for it - you have to kill an xpac boss - first boss in UK seems to be the easiest and you don't need it set for heroic, meaning your group can pop in, kill, reset (he only drops one) rinse and repeat.
The bouquet is right clickable and produces the same effect as throwing a handful of pedals at someone - need to confirm if using this will give you credit for the throw pedals at [list of race/class combos] achievement.
Edit 2: Confirmed - Ebon roses wont work.
People have also been selling the black dresses on the AH - don't buy them - it doesn't count for the achievement.
Like the dress, you have to pull the candy out of the bag yourself - you can't trade it so don't buy it and don't waste time mailing it to friends/guildies/alts.
The Love Fools only last for 15 seconds (per mouse over info) so don't drop it till you're ready to use it - you can share and use other people's fools though.
You don't have to be in the castle in WG nor does your side have to have control to pity the fool there - just make sure that the location bar above your mini-map says WG and you're set.
Edited, Feb 11th 2009 11:11pm by rusttle
Edited, Feb 12th 2009 1:18am by rusttle