1... you could spot useless xy-wings ( which are xy-chains ) and try and extend them
2... always make sure the cells you are trying to string together are bi-value (contain only two candidates).
3...each end of the chain, when given a value, will be true in both directions for that value... this gives the ends of the chain a "pincer" characteristic... the same as a xy-wing (which is a xy-chain).
in other words, its true in both directions and the same value is true on each end, therefore any candidates those two ends see must be false.
AB---BC---CD---DA
A(B---B)(C---C)(D---D)A.,.,.,.,.four cell xy-chain, obviously there can be longer chains.
here is the xy-wing
AB---BC---CA
A(B---B)(C---C)A
so as you string together xy-chains, the value that you didn't use in the previous cell must occupy the next cell that it "sees".
for example... hypothetical situaion...
start with {1,2} then you would look for a bi-value cell that this cell can see containing either a 1 or a 2.. eg... {2,3}...since you used the 2 to connect them, you must use the 3 to connect the next bi-value cell...{3,4}... then use the 4...{4,1}... the 1 in this last cell as well as the 1 in the first cell would act like pincers and eliminate any other 1's they see. obviously all of the xy-chain cells must "see" the next cell in the chain to work.
(12)-(23)-(34)-(41)
1(2---2)(3---3)(4---4)1
notice how the 1's are left out, creating the "true" on each end characteristic regardless of the direction you travel through the chain.
visual example...
3(4-----4)(9-----9)(1-----1)3
the 3's are left out A.K.A. true on each end and eliminates the 3's they see.
(3=4)r7c2-(4=9)r5c2-(9=1)r5c3-(1=3)r4c3; r79c1<>3
