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The "Skilled Pact Making" feat (page 74) grants a +4 bonus to binding checks, but requires a base Will save of +4 (which you'll reach as a 4th-level binder, meaning you can take the feat as your 5th-level feat [if using PF rules] or as your 6th-level feat [if using 3.5]).
In addition, the "Implements of Binding" item (page 76) grants either a +1 (930 gp), +2 (1530 gp), +4 (3930 gp), +6 (7930 gp), or +8 (13530 gp) bonus to binding checks.
So, at binder level 5/6 with a 16 Charisma, the feat, and the +2 item, you'll have +14/+15 to binding checks, and thus automatically succeed at a DC 15 check.
If multiclassing, you can achieve the same at lower binder levels, by purchasing one of the greater versions of the implements of binding.