Well, I've been doing all the Valentines achievements and noticed on various blogs some hints and tips, but thought I'd write some of my own as none of the guides seemed to cover it all as far as I can remember. This is Alliance biased as I don't have a Horde character.
First put on your Perfume (to talk to Male chrs - Most if not ALL the IF guards are male) or Cologne (for Female chrs - almost all if not ALL the Darnassus Sentinels are female). You can buy these from Innkeepers - get some love tokens while you are there. Get LOTS.
Pledges come from Guards (SW or IF) or Sentinels (Darnassus). You can get a pledge of Adoration once an hour, which contains one of the rare items (picnic basket, trueshafted or Silver shafted arrow, lovely black dress, love rocket, love fool, bag of candies, box of chocolates, handful of rose petals, unbestowed friendship bracelet). You always get a Card or Pledge of loyalty as well.
Gifts come from normal NPCs such a bread vendors. Again the "of Adoration" can contain anything listed above as well as one of the other items you need such as bread, brew or wood crafts (depending on the City). Be careful in IF - one if the wandering NPCs is actually affiliated to SW and will give bread not beer.
You can obtain the "normal" items any time - these are Cards, pledges and crafts. If you collect 5 of each thing - card, pledge and craft/food/beer you can combine them all to make a gift collection for the particular city (and get a buff for doing it - Agility in Darnassus, Intellect in SW and Stamina in IF - you can't have more than one buff at a time though).
If you collect all three city Gift packs you can create an Alliance gift pack which you take to Peddlefeet (he's in the King's room in IF). If you hand it in you get a reward of a Box of Chocolates and one of 5 of Arrows, Love rockets or Handful of petals. Give him a /kiss and you will receive a +200 health buff as well.
This means that you don't have to wait to receive all the items in your adoration once an hour thing. You can get your chocolates, rockets, arrows and petals by other means.
The Love fools - again you don't have to get these, but you will need a friend with them. You can /pity any fool that is in the location required, so if you don't get them in your gifts but a friend does you can follow them along for that part.
You can trade the lovely dress, fools, rockets, petals, pledges, cards and crafts/beer/food, but NOT arrows, chocolates, picnic basket, truesilver arrow or candies.
For the flirt with disaster achievement you need to remember alcohol to get completely smashed and your petals to throw on him and then you have to /kiss him. He is usually found wandering around the middle circle of Ironforge (surrounded by people).
You can follow your own way about the quest from the guy that leads to the coloured dresses or suits... it ends up out the back of beyond in Arathi - I've done it every year for 3 years, but I still struggle every year to find it!
Shoot people with arrows - easy enough. Cure people's hearts - Darnassus is good because there's never anybody there - stand by the bank or up the top of the ramp in the way out. In the square by the AH in SW is also good, as is the bridge in IF.
Dalaran is a good place to find the class/race combinations you need to throw petals on.
The easiest place to get a bunch of ebon roses is off the first boss in Utgarde Keep. Take a group and run it over and over until you all have what you want (it only drops for one person). Or do like I did and have such wonderful guild mates that they know you do mad things and want strange items that they will come with you JUST for killing the boss - /hug guys. I LOVE MY GUILD.