Originally there were two issues in relation to getting any of the four collectible achievements linked to the Claptrap DLC.
The first one was a bug that is stopping a number of the collectable achievements:
A: “What a party!”
B: “It’s so realistic”
C: “Bobble-trap”
D: “The Lubricator”
Popping when they are achieved. Gearbox released a patch to fix the problem in late 2010 so this is no longer an issue.
The second issue is that the drop rates for a number of these items are obscenely low.
The best way I have found to deal with this problem is to "actively farm" MINAC - the final boss at the end of the DLC.
Now, many people are getting to MINAC, going to hide in a corner where MINAC and the Claptraps it releases can't get at them, and going away to do something else. After 3-4 hours they come back, kill MINAC and collect a tonne of items from around the area.
This I consider to be "passive farming". I have found, after much testing, that "actively farming" MINAC is immeasurably more rewards and prompts drop rates 3-6 times better than "passive farming" on all items.
"Active farming" involves dying a lot, to be blunt. But you aren't losing money when you do so, and MINAC doesn't return to full health if you do so (unlike Crawmerax) so there's no disincentive in dying.
To "actively farm", you need to shoot out all of MINACs turrets - not his "eye", but everything else. Once that is done, you can attack a bit more confidently.
Follow MINAC when he is on the move, but stay behind it so the "eye" can't get you. When MINAC stops, it will release Divine Wind claptraps - and this is when you attack.
Actively kill the claptraps from close to medium distance. What happens when you do so is that MORE and MORE spawn from MINAC.
For example, if you get in and kill the first tow claptraps to emerge from MINACs doors before those doors close, more claptraps will spawn. If you kill them, 2 more will spawn. And so on.
What happens is that instead of 2 claptraps emerging from the doors every time MINAC stops, 6-8 emerge.
Also, I found that with these extra claptraps emerging (and the greater number of dropped items which naturally appear), actively hunting them down seems to increase the chances for special drops.
In 90 minutes last night, I gathered 10 sets of 3D glasses, 6 Clapfish, 8 pink panties, 6-8 oil cans, 6-8 bobbleheads and 5 pieces of pizza. In just 90 minutes!
Once you're finished farming, kill MINAC and gather the rest of the items. But yes, get out, chase the claptraps and actively farm - its more fun and hugely more profitable.