I used to do this a few years ago with this procedure:
5X ligation Buffer:- 250 mM Tris.Hcl (pH 7.6)
- 50 mM MgCl2
- 25 % PEG 8000 (from Sigma # P2139)
- 5 mM ATP
- 5 mM DTT
Use one unit of T4 DNA ligase and a ratio Vect/Insert ends=3
Ligate a few hours @ RT, or alternatively begin @ 15°C in a cold bath, and let it reach RT Overnight.