For PLA, white marks disappear at just over 60°C. A warm water bath works great, as does holding it over boiling water (use tongs!)
My favorite technique is to hold the part with tongs and wave it in front of the steam from a boiling tea kettle. It takes no more than a second, so you can do this on a dozen parts in rapid succession.
If you do this quickly, the plastic won't even soften noticeably; the white marks should disappear instantly.
If you don't want to get it wet, a heat lamp should also suffice. Holding it over a toaster for a second should work too. PLA gets soft at such a low temperature there's no need to use a heat gun, open flame, or soldering iron.