Heres my tip, well there will probably be more but oh well.
Be self Sufficient
By that I mean: fish and cook your own food, use runes you get from slay drops in mage, or even make them with ess you get slaying, make your own pots from herbs you get slaying (or farming, or kingdom), fletch you own bolts/arrows, smith is needed too with bolts.
Doing stuff like this will save you mills in the long run (99 Slay is a loooong journey) and most of the skills needed to do this can be assisted. lol, you can ask me for assist even. Try to be self sufficient in one or two things at first, then try to broaden the spectrum. Doing this will even net you well needed xp in less used skills such as rc, fletch, fish and cook.
The one downside to this is that it obviously takes more time.
When you find yourself fishing though, don't sell the fish, cook and use them while slaying. You got a ton of herbs from a spec task? Don't sell em, make them into the pots you use slaying. You get the idea, and trust me you will save a ton!