What precautions would you take before trying heroin?

I'd prepare a schedule of upper-bounds on usage, including a scheduled, externally enforced hard stop date and detox period. For milder drugs, I use a "day, week, month" rule - one day out of every week, one week out of every month, and one month out of every year the drug is not allowed. For something as addictive as heroin, I'd strengthen this considerably - probably including a 3 month scheduled no-use period to extinguish the habit, just long enough after starting to get the data on whether it's resuming after the 3 months. And I'd set things up such that if I broke my own rules, my friends would notice and intervene. (Note that this means the withdrawal periods have to be counted on the 'cost' side of a cost/benefit analysis, and that almost certainly means not using strongly-addictive substances at all).

In the case of World of Warcraft, I'd set a scheduled uninstall-and-cancel-subscription date, then resume months later. I believe they offer a free trial period, the end of which would be a natural time to do this. (But be sure to keep repeating the detox periods!)