Forget all that talk about corps and quests and journals and whatever. Yes, they can all be very useful, but there is a FAR better alternative: Making Molotov Cocktails.
I kid you not. This new training method, patented under the name of Poltifar's Patented Sure Fire Molotov Cocktail Making Training Method (PPSFMCMTM for short (I dare you all to try pronouncing that)), gives a SHITTON of XP for very little resources and approximately NO danger to one's self (unless you actually try to actually use the actual molotovs you make, in which case refer to section 104A-44F for a disclaimer before trying to sue me for my PPSFMCMTM's dangerousness). Normally, it doesnt give actual money, unless you manage to find a player willing to buy them in the hundreds to grind his bomb skill or something. But still, SHITTONS OF XP.
Basically, each moltov needs one t-shirt (ANY t-shirt) and a bit of alcohol. Get a jerrycan, fill it with alcohol from the bathtub in crack mansion, get alot of t-shirts from orphans or crackheads as you kill them as usual, and go to your appartment (Were you would have the required chemistry set(not included in the purchase of the PPSFMCMTM, sold separately in a hobby shop near you!), and make 100-200 molotovs. EACH MOLOTOV GIVES 40XP, NO MATTER HOW HIGH IS YOUR TOTAL XP (atleast, it has been a constant 40 each for me up to my current 14.5k xp, not sure if it stays like that indefinitely), so making 200 of them gives you 8000 XP (dont think that 200 is such a high number, i made 150 in 2 days, netting me a total of 6000 XP AND TWO WHOLE NEW MUTATION SLOTS).
Also, another disclaimer: I do not take responsibility for any personal harm that might come from misuse of the molotovs or people trying to make them with less than 14 total chemistry (if you have under 14 total chemistry and still want to make some, get fireproof clothes. YOU'LL NEED THEM. ALOT.)
So, bored of killing stuff? Annoyed of being hurt and killed by ennemies? Or maybe ennemies dont even give enough XP anymore? Have tons of shirt lying around? You are a true dwarf with barrel upon barrel of explosive alcohol? Have high chemistry or brain, or want to grind chemistry or brain? Heck, just want to blow shit up with molotovs while getting free XP?
Try Poltifar's Patented Sure Fire Molotov Cocktail Making Training Method today!