I have always wanted to find out this answer... Other than the start to end services, i.e 8 28 31, here are a few alternatives.. Some of which have been mentioned already.. They will not be ranked in any way.
BUS only.
TAM - 23 to Bendemeer Rd (Blk 54), then 26 to TPY. Alternatively, take 61 107 853 from Bendemeer Rd (Blk 54) to Serangoon Rd (St Michael's Place) then take 26 or 31. This is to increase your chances of getting to TPY faster.


TAM - 23 to Jln Kolam Ayer (SCDF), walk under Woodsville Flyover then take 8 or 90 from Macpherson Rd (Opp POSBank) to TPY.

TAM - 22 or 65 to Eunos Link (Blk 312), 93 to Braddell Rd (Blk 219), then 28 73 105 153 155 159 238 to TPY/TP MRT. Alternatively, take 8 from Eunos Link (Blk 312) towards TPY.

TAM - 22 or 65 to Bedok Reservoir Rd (Majority except 2 pairs between BN Ave 3 and TAM Ave 1), then 506 to Lor 6 Toa Payoh (TP Swim Complex), walk to TPY. Alternatively, take 21 from Tampines Ave 4 (TAM MRT) to Bedok Reservoir Rd.

15 from Tampines Ave 4 (TAM MRT) towards Jln Eunos (Opp Eunos MRT), cross the road and take 966 to Jln Toa Payoh (Opp Blk 195), then 8 26 31 90 or 142 to TPY.

TAM - 18 to BDK, then 26 to TPY.

TAM - 72 to Tampines Rd or Hougang Ave 3 (Blk 21) then take 81 to Upp Serangoon Rd (KVN Stn) then take 153 to Lor 2 Toa Payoh (TPY MRT)

TAM - 72 to Hougang Central (HGN) then take 153 from HGN Int to Lor 2 Toa Payoh (TPY MRT).
Bus and MRT
MRT: TAM - BDK, then 26 to TPY

18 from TAM to BDK, then MRT: BDK - KAL, then 26 or 31 from Lor 1 Geylang (Blk

towards TPY. Alternatively, MRT: BDK - PYL, then 28 or 155 to TPY from Paya Lebar Rd (PYL MRT).

MRT: TAM - KAL, then 26 or 31 from Lor 1 Geylang (Blk

towards TPY.

MRT: TAM - PYL, then 28 or 155 from Paya Lebar Rd (PYL MRT) towards TPY.

MRT: TAM - EUN, then walk to Jln Eunos (EUN MRT) to take 966 to Jln Toa Payoh (Opp Blk 195), then 8 26 31 90 or 142 to TPY.
I hope that answers MOST of the solutions.

If my service 20 comes out, just take it from Tampines Ave 4 (TAM MRT) and alight at Lor 6 Toa Payoh (TP Swim Complex). That would be cool!

Note: my service 20 does not exist. For more info about my service 20, go to
http://buses.sgforums.com/?action=thread_display&thread_id=67711&page=0.