Maybe things have changed since the time when it was hard to level lockpicking, but I have personnaly found it very easy (I am currently at 270).
I used the information above to level until about 220 and then only did I realize that I could effectively pickpocket all humanoids that have pockets ;) (and some non humanoids as well, like the slime-like monsters which btw look like they have tons of things in them as you can see arrows and other items in them).
SO, pretty easy: before using cheap-shot/ambush on any monster, pickpocket it first (mobs tend to detect you extremely rarely if you do it properly, i.e. coming from behind). You mostly get money, sometimes other cool stuff (namely rogue components) and sometimes chests. These are not as common as the other stuff, but still fairly common. If you do that on mobs of your level, you'll get green/yellow/orange chests, so not each will guarantee a skillup but most will give you one. According to my experience, about 10 chests will do for the 5 skillups you need at each level. And getting 10 chests from lockpicking is very easy. That's it.
Hope this will help others (try to be smarter than me and do it from the very beginning, you can occasionally also drop some cool stuff from lockpicking).
N.B. This lockpicking habbit helped me a LOT with some quests where you have to drop items from mobs... especially in Ashara when you have to drop scroll pieces from the slime-like creatures since a) there are very few of them and b) the drop-rate is ridiculously low: I pickpocket MORE pieces from the mobs than I got from looting them... and sometimes I got 1-2 from pickpocketing AND 1-2 from looting... SO, always pickpockets mobs from which you are expecting a specific loot.
N.B. bis: This is a bit obvious, but I still wanted to reply to one poster who said that, strangely, the doors of the Scarlet Monastery were green to him with about 170 lockpicking or something: this is because you character carries the key to the monastery, thus it is green. Otherwise it would be red.
Cheers,
nostra