To get a sub 15 average- its quite easy really, You just need to do 5 solves that average <14.99!
In all seriousness F2L lookahead is what really gets one sub-15 in my opinion assuming everything else is decent.
Some alternatives that might also help you get sub 15 if you don't like F2L, in order of Feasibility decreasing
(1) Cross practice + learn how to X cross, this can really help. Maybe try Roux to practice block building for X crosses.
(2)Learn full OLL + PLL (if not already done) and sub 1 Every Alg. Pros: makes last layer sub 3. Cons (may take a few months, Plus dem F perms be hard to Sub 1.)
(3) learn some VHLS easy cases, COLL, VLS, WV etc cons (won't help a ton).
(4) learn FULL ZBLL Pros: Sub 2.5 LL assuming your EO during F2L is real fabulous. Cons: (may take 3-5 years to fully master)
(5) FULL 1LLL. Pros: 2.5-2- LL Cons (may take 12-20 years to master so better start young)
(6) OPAL (Orient+Permute All layers) Pros: sub 2 entire cube Cons: ~43 Quintillion Algs so better get a computer brain.
(7) Make some Rice Pudding-Southern hemisphere style and hope for the best. Cons: If you are allergic to rice this is a bad idea
(8) Hope for a F2LLL skip. (F2L + LL skip). Step 7 may assist with this