Answer

By Country Walking magazine

We'd recommend the old-fashioned double (or triple if they're really slippery) bow before investing in new laces. Still coming undone? Try flat laces rather than round - we don't know the science bit, but in our experience they just seem to stay done up for longer.  

Alternatively, try the curious looking Xtenex laces recommended by ptc on the forum (see pic). They have bumps every few millimetres to hold the laces in place, which magically disappear when the laces are stretched so you can actually get them into your boots. We got hold of some to test - it was a bit of a tussle to lace them up, but those bumps kept the laces securely tied where regular ones worked undone.

And they don't just stay done up, they also allow you to adjust the tension of the lacing across the boot to perfect the fit, like wearing them loose over the forefoot if you have wide toes, and tighter round the ankle to keep them secure. At £8.99 a pair they're not cheap but we reckon they're worth a punt. Buy online at Runner's Retreat or Ten Point.

Update
Many thanks to LFTOer Richard66447 who got in touch to say his local cobbler advised it wasn't the shape of the lace which was important  but the amount of wax in them: the more wax, the more they slip. The cobbler suggested a round pair that didn't have much wax (offering a full refund if there were problems) - but they worked a treat and stayed done up.