Sideboards always include ways to fight the most common graveyard hate in the format. Sideboards will vary depending on the expected metagame but some cards, like Ashen Rider, are almost always included.

Creatures: Keep in mind that if you are sideboarding in Legendary creatures Karakas becomes a very strong card against us.

Ashen Rider is a good option for a Dread Return target because it deals with many problem permanents like Glacial Chasm, Ensnaring Bridge, Griselbrand, and more. The fact that the Ashen Rider gives you another exile ability when she dies gives you incentive to even use her to pay the Flashback cost of Cabal Therapy or another Dread Return.Elesh Norn, Grand Cenobite is a trump card against creature strategies like Merfolk, Elves, Delver, and other Dredge players.Iona, Shield of Emeria is a trump card against many decks that heavily rely on one color to win. Examples of these types of decks are obvious one color decks like Death & Taxes (although they can still use Aether Vial to flash in creatures), Elves, and Merfolk. Multi-color decks like Storm, Show and Tell, Infect, and Miracles can also be seriously hindered.Faerie Macabre has utility against opposing graveyard strategies and the fact that it is a black creature means it can be exiled to Ichorid. Because Faerie Macabre’s value is found in its activated ability, it costs us no mana and cannot be stopped by standard counterspells like Force of Will.Ingot Chewer is good against artifact hate for our strategy and has the benefit of triggering our Bridge from Below for zombie tokens because Evoke causes it to die after entering play. It is only usable at sorcery speed though, which is a disadvantage.Wispmare is the anti-enchantment equivalent of Ingot Chewer.Street Wraith is a decent choice against individual card graveyard hate like Surgical Extraction and Extirpate if they target a dredger because you can just dredge that card in response. Otherwise, Street Wraith can be helpful in accelerating the deck by practically giving us a free dredge. Worst case scenario, it can be exiled by Ichorid or reanimated by Dread Return. While this card has potential it is not generally considered an optimal use of a sideboard slot.
Instants & Sorceries:

Chain of Vapor is an interesting card because it allows your opponent the chance to turn it back on you somewhat. However, it is still a strong temporary answer to permanent graveyard hate like Rest in Peace or Grafdigger’s Cage. Of course, in many scenarios a temporary answer is sufficient because you can bounce the permanent on your opponent’s end step and then win on your own turn. Chain of Vapor has excellent synergy with Cabal Therapy because it gives you a known target in the opponent’s hand.Nature’s Claim is an excellent choice for a flexible anti-hate card. It is good at killing your opponent’s Rest in Peace or forcing an activation of Tormod’s Crypt on their turn so you can start dredging again on your own turn. The drawback is giving your opponent 4 life (if you force the activation of single-use hate like Nihil Spellbomb Nature’s Claim becomes countered by the lack of a target and your opponent does not gain life), but that will rarely decide a game because opponents will often keep a hand with graveyard hate and nothing else.Natural State fills almost the same exact role as Nature’s Claim but has one drawback that makes it a risky choice: it cannot kill Leyline of the Void, Moat, or similar 4+ cost permanents.Abrupt Decay is a strong option for the sideboard. It has a lot of potential because it cannot be countered, and it is also flexible in that it is able to kill both enchantment hate like Rest in Peace, artifact hate like Relic of Progenitus, and creature hate like Containment Priest. The main drawback is that you must devote two mana in one turn to cast Abrupt Decay.Firestorm is excellent against creature based decks and hate because not only will it remove creatures, but it allows you to discard cards as well. This is the perfect example of a card that enables our strategy as much as it hates on competing strategies. Note, discarding cards is part of the cost of casting Firestorm, therefore, even if Firestorm becomes countered, the cards are still discarded as the spell goes on the stack. Another trick is to target both players even if there are no creatures on board, therefore allowing you to discard twice regardless without any conditions.
Others: Sometimes lands are also included in the sideboard to help with the issue of casting anti-hate cards in games 2 and 3.

City of Brass would usually be the land of choice to include in the sideboard if there are not already four in the mainboard.Lotus Petal performs a similar function as extra lands do in the sideboard, except it is possible to have your Lotus Petal countered. Lotus Petal does allow you to play more spells on one turn than you could with only your land drops, a quality that can be extremely valuable in racing the fast combo decks.Pithing Needle is excellent against activated graveyard hate, including Deathrite Shaman, Relic of Progenitus, Scavenging Ooze, Karakas, and more. It stops multiple copies of a single card from being activated, a nice bonus.Leyline of the Void is of course very useful against other graveyard decks. One nice bonus is that Leyline of the Void stops your opponents’ non-token creatures from entering the graveyard, and activating your Bridge from Below’s exile trigger when they die.