Tips/Advice for a Playthrough of Pokemon Heartgold/Soulsilver?

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

You can get a bicycle in the Miracle Cycle shop in Goldenrod City.

The best HM slaves are probably Krabby (rock smash, cut, strength, surf, whirlpool), Farfetch'd (cut, fly), and Golduck (rock smash, strength, surf, whirlpool, waterfall).

The game gets a bit non-linear between Morty and the Radio Tower. You can clear the Rocket HQ and battle Chuck and Jasmine in whatever order you want. You must battle Pryce after the Rocket HQ, but you can still do it before the other gyms.

You can get an exp. share from Mr. Pokemon on Route 30 by exchanging the red scale after battling the red Gyarados.

Heart scales are kind of rare in this game, so once you reach the move reminder in Blackthorn City, you might want to look up their locations on Bulbapedia and pick up all the ones you missed earlier. If that's still not enough, you can smash rocks in Violet City or Tohjo Falls.

Daily events reset when the clock advances from 23:59 to 00:00, so it's easy to abuse this mechanic if you're willing to mess with your device's clock.

Useful HMs, TMs, and held items and their locations
TM89 U-turn: Azalea Town (received upon defeating Bugsy)
TM14 blizzard: Goldenrod Department Store
TM25 thunder: Goldenrod Department Store
TM27 return: Goldenrod Department Store (Sunday)
TM38 fire blast: Goldenrod Department Store
TM52 focus blast: Goldenrod Department Store
TM57 charge beam: Goldenrod Department Store lottery (Wednesday), Olivine City (requires surf)
TM60 drain punch: Goldenrod Department Store lottery (Thursday), Route 39
TM65 shadow claw: Goldenrod Department Store lottery (Monday), Route 42
TM91 flash cannon: Goldenrod Department Store lottery (Tuesday)
You can soft reset for the lottery if you save before talking to the person.
Metronome: Goldenrod Game Corner
TM75 swords dance: Goldenrod Game Corner
TM28 dig: National Park
HM03 surf: Ecruteak City theater (received upon defeating the Rocket Grunt)
TM30 shadow ball: Ecruteak City (received upon defeating Morty)
TM36 sludge bomb: Route 43
Choice specs: Lake of Rage (not Wednesday, requires surf)
HM02 fly: Cianwood City (after defeating Chuck)
TM23 iron tail: Olivine City (received upon defeating Jasmine)
HM07 waterfall: Ice Path 1F
TM26 earthquake: Victory Road 2F (accessed from the middle of 3F)
TM79 dark pulse: Victory Road 3F

Once you defeat Lance, none of this advice is guaranteed to work. You're on your own.