Tips/Advice for a Playthrough of Pokemon Red/Blue/Yellow?

Question

A thread for posting (spoiler-free) advice for a playthrough of the given game which will help anyone playing it. The aim of this thread is to make playthroughs easier for someone who is playing it for the first time.

A couple of rules:

1. Don't give advice for Pokemon as a whole. Seperate thread for that can be found here.
2. Try to keep advice beginner friendly. Do not go into the more complex mechanics.
3. Don't post multiple answers. Edit your existing answers to add more.
4. No repetition

Answer 1

Be sure to battle your rival on Route 22 for a bit of experience before battling Brock. Once you defeat Brock, the rival won't be there anymore.

If you have a water Pokemon like Blastoise or Vaporeon, the best HM slave combination is probably Jigglypuff (flash and strength) and Farfetch'd (cut and fly). If you don't have a water Pokemon and you do need a surf user, you can replace your Jigglypuff with a Psyduck or Slowpoke or something after you get the super rod on Route 12. Both Psyduck and Slowpoke can also learn dig, if that's something you want.

You can get the bike voucher from the Pokemon fan club in Vermilion City and then exchange it for a bicycle in Cerulean City.

Some Pokemon have high attack and no physical STAB, or high special and no special STAB. In some cases, you'll want both a STAB move and a move for its stronger attack stat, because either one can do more damage depending on your opponent's types and defense stats. In other cases, you have a Haunter. Do not try to teach STAB moves to a Haunter.

Do not use the exp. all. If you have it, deposit it in your PC. Because of a glitch, it causes your party to gain less total experience in some situations.

RBY is less linear than some of the other Pokemon games. If you want to make a gym leader or any other boss easier, explore as many routes as possible and defeat the trainers and pick up the TMs there. It's much faster than grinding with wild Pokemon. You can also do the boss battles themselves out of order, like Sabrina before Koga, if you want.

Useful HMs and TMs and their locations
TM28 dig: Cerulean City, received upon defeating the Rocket Grunt
TM45 thunder wave: Route 24
TM08 body slam: SS Anne 1F (permanently missable, be sure to pick it up before leaving the ship)
TM24 thunderbolt: Vermilion City, received upon defeating Surge
TM13 ice beam: Celadon Department Store roof
TM48 rock slide: Celadon Department Store roof
HM02 fly: Route 16, house in northwest corner
TM10 double-edge: Rocket Hideout B3F
TM29 psychic: Saffron City, Mr. Psychic's house
HM03 surf: Safari Zone area 3, secret house
HM04 strength: Fuchsia City, Safari Zone warden's house upon returning gold teeth
TM03 swords dance: Silph Co. 7F
TM26 earthquake: Silph Co. 10F
TM14 blizzard: Pokemon Mansion B1F
TM38 fire blast: Cinnabar Island, received upon defeating Blaine

Answer 2

Don't use Focus Energy or Dire Hit, as they divide the user's critical hit rate by 4 instead of multiplying it by 4.

Critical hits ignore all stat changes. This can result in a critical hit dealing less damage than an attack that does not land a critical hit. Use this to your advantage; you can use Pokémon with High Crit rates, or guaranteed hit rates, to bypass the opponents stat changes that would put you at a disadvantage. Additionally, if you have a Pokémon with a high-speed stat, don't rely on stat buffs, as high speed = more crits that aren't affected by the buffs.

Like states above, Pokemon with high speeds get more crits. This can be stacked with moves that have higher crit chances. HIGH SPEED + HIGH CRIT RATE = LOTS OF CRITS!!! Even better if you can get Same Type Attack Bonus, STAB, too!
list of crit chance per Pokemon

Multistrike moves dealt the same amount of damage for each hit in a turn, meaning that if the first hit was a critical hit, the other hits would be critical hits as well.

You can use a Pokedoll to bypass the Ghost Marowak if you don't want to get the Silph Scope.

Mimic is a very useful move, as you can select which move you want to learn from the opponent, and that move is temporarily added to your moveset. Use this when you're stuck somewhere, if someone has a move on their party that would be super effective against them.

Use the Badge Boost Glitch to your advantage. Attack is boosted by Boulder Badge (Brock), Defense by Thunder Badge (Surge), Speed by Soul Badge (Koga), and Special by Volcano Badge (Blaine). Every time you increase your stats in battle, these boosts get added again. You can abuse this.
Badge Boost

All and all, Poliwag's 90 base speed makes crits happen 17.5% of the time. That can be viewed as slightly less than a 1/5 chance. Double Slap packs a punch with this little guy, considering the early Attack boost from Brock and 2x damage from crit. Highly recommend this little guy early on.

Tl;dr, use fast Pokemon :) if you don't want to use speedy boys, use slow boys + stat boosting moves to abuse badge boost glitch for extra buff.

Comments

I somehow completely forgot about critical hit mechanics while writing my answer. Multi-strike moves still do the same damage as other moves on average (and have lower accuracy), because if the first hit isn't critical, this also prevents the other hits from being critical. I think Sandslash and Dugtrio are better critical hit abusers because they have higher base attack stats and can learn slash.