Some of this has probably been mentioned before, but I still want to list them all together.
First of all, as you are leveling a warlock it will not take you more then 10 days of playtime to reach level 70 if you are a decent player. And if you level by questing, I suggest, that you should, like me, SKIP PET SPELLS.
What I mean by this is that you don't buy every single rank of a felhunters devour magic but only the highest one once you are level 70, as that is possible. You just have to buy the highest rank (level 64 one) and you can skip all the other ones, unlike with normal spells as shadowbolt which you do have to buy rank 1, 2 , 3 , 4 etc of.
This will benefit you while leveling because we only level faster nowadays, so probably get less gold while going to 60 which results in not having a epic mount directly at level 60.
This goes for the spells:
felhunter: tainted blood (actually useless at all), and devour magic (just buy level 64 one), spell lock and paranoia only have one rank
succubus: lash of pain (6, soothing kiss (70 , actually also useless), seduction and lesser invisibility only got one rank.
voidwalker: consume shadows (66) , sacrifice (64, pvp under level 61 is kinda useless anyways :P), suffering (69), torment (70). You may want to buy them all because you level with a affliction voidwalker spec/ strategy, else just buy the highest rank.
Imp (my favourite pet with affliction spec): Get firebolt on 68 whatever spec you are, you won't ever use it and it only wastes ur imps mana, fire shield (64), blood pact all the way, unless you don't level with an imp get it at 62, and the only one rank of phase shift is at level 12.
If you have a felguard, and level with a felguard, which probably go together as the sl / sl spec is used much more often nowadays in pvp, you should get every single rank of the spells as you will just need it while leveling.
Now there is a second big part where you can save gold while leveling:
FORGET FIRESTONE, CURSE OF RECKLESSNESS, CURSE OF WEAKNESS, EYE OF KILLROG, SPELLSTONE, HEALTH FUNNEL (unless you level wit a voidwalker or felguard), AND CURSE OF ELEMENTS (unless you are some retard fire destro leveler :P).
all of these exept fire and spellstone, and imo also curse of recklessness and curse of weakness you should just forget and these spells should be replaced by usefull spells but thats me. All of the others I just mentioned are barely used until level 70 so just train them at level 70 when you can make easy gold.
Now the last part of my ''saving gold as a leveling warlock" guide, is surely mentioned before. Though just to tell people who still don't know it again:
Do NOT buy J'eevee's Jar (150 gold), Black Lodestone (50 gold), and Xorothian Glyphs (50g) for your dreadsteed quest once you are at the point of the quest in which you should. Another warlock who did buy them can use them for you in dire maul west to summon the event which will save you 250 gold.
Now for the playtime saving tips:
If you are horde, do the orc succubus quest over the bloodelf / undeath succubus quest, it will save you ALOT of travelling and by that also playtime. This is possible , whatever race you are, just go for the orc one. This one is all around the barrens and just a little in south stonetalon, but the undead one lets you go to halfway wetlands (takes hours to get), and to southern silverpine forest (also not a place you come every day).
Think there ain't much more unique things I can share, hope these help
If some of it is unclear or formulated terribly wrong I am able to explain in a reply.
GL AND BAI!