I ride everyday in pretty much all weather (-3C & frosty off the main roads this morning, lashing rain this evening, still found a few opportunities to filter on both journeys)
Keep switched on & have a constant scan going on, ahead is much more important than behind, if you see a gap ahead assume someone will try to cut across into it.
The same as overtaking don't go into it blind, if you don't see somewhere you can merge into, or see ahead to where stopped traffic is beginning to move don't go. If you can see traffic ahead beginning to move you should be able to poach a gap.
Depending on how traffic is moving you're probably better off between lanes 1&2 (on a 2 lanes each way road) than between 2&3. When both directions are stopped & if there's a suitable gap I prefer being between 2&3.
Once traffic is moving above 20mph or so I generally pick a point & get in lane.
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Joshorilla It's generally safer to be out of the way of anything that weighs 20 times my weight. That means not sitting waiting to end up squished between 2 cars. Cage drivers are generally ignorant of how bikes behave & that they can filter in a safe manner. Claiming it's dangerous so adamantly when you've not done it & not shown any evidence gets you lumped in with the cagers.
It's odd how people who don't filter are so quick to claim it's stupidly dangerous, but anyone who rides a bike where it's legal sees it as fine & safe.
Out of the dozen in my group at a post test assessment & training scheme run by the local cops all bar one filtered on a daily basis, the 1 guy who didn't was only back to bikes after 20 years off them & had no confidence on the road.