If you’re working in Kdenlive, guides are your friend. As are the razor, slip, and ripple tools (take a moment to really learn them and you’ll save yourself a lot of time).
Also, the alt+arrow keys for navigating between clips on the timeline and ctrl+arrow keys for navigating between guides.
Finally, useful filters: Everything in Motion, Volume and Dynamics (audio fades, etc.), Alpha, Mask, Keying → Obscure (pixellate sensitive stuff).