(This might not be updated, since I change up the sideboard often)
As a general rule, I wouldn't go below 40 lands. If you KNOW the matchup will be really grinding and there is absolutely no hope of comboing T4 then you can probably take out more. Depending on the matchup, it would be smart to either board out a Blast Zone, Gemstone Caverns, or Reliquary Tower.
Also, this sideboard is based on the current competitive meta. If your local meta is different, then you should definitely make appropriate changes.
New: Noxious Revival - this is very good against hand disruption, counter magic, and permanent destruction. I have used this card in so many ways, and you can even effect your opponent's graveyard and draws with this.
New: Void Mirror - For decks like Living End, Rhinos, Tron (sort of), pitch Elementals, Bring to Light. I am trying out 3 right now because of how heavy the competitive meta has these decks. However, if you are only playing at a local LGS, then you can swap this for something else.
Shattering Spree - Artifacts suck. Like Pithing Needle, Lantern of Insight and other control artifacts, and Tron artifacts. They all gotta go. Also great against Urza's Saga decks.
Laboratory Maniac - Yes, I love my Leyline, but oh it can hit me hard too. So now I can mill myself! This is also a good way out of a lot of controlling situations too. Lifegain can be tricky to beat, so this helps out there.
Damping Sphere - Tron (rough match up) and Storm. Also hurts other combo decks that are pretty much decided by whoever goes first.
Defense Grid - control ¯_(ツ)_/¯
Leyline of Sanctity - picture Jund/Shadow, then picture this deck. Yeah, not a good picture. (I mainboard 4 of these in more competitive scenarios btw, because it helps the chance of winning Game 1 against Blitz, Burn, anything with hand disruption, Storm, Heliod + Walking Ballista, Valakut, and so on.)