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contours = DeleteDuplicatesBy[First]@
  Join[{{-1, Directive[Thick, Red]}, {0, Directive[Thick, Green]},
        {1, Directive[Thick, Blue]}}, 
   Thread[{FindDivisions[{-2, 2}, 10] , Automatic}]];

ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
 PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
 ColorFunction -> "Rainbow", ImageSize -> 500, 
 Contours -> contours]

contours = Cases[cp, Tooltip[_, t_] :> t, All]

{1.5, 1., 0.5, 0., -0.5, -1., -1.5}

styledcontours = Join[ Thread[{{0., -1., 1.}, 
     Thread[Directive[{Green, Red, Blue}, Thick, Opacity[1]]]}], 
   Complement[contours, {0., -1., 1.}]];

ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
 PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
 ColorFunction -> "Rainbow", ImageSize -> 500, 
 Contours -> styledcontours]

automaticcontours = FindDivisions[{-2, 2}, 10];

styledcontours = {{-1, Directive[Thick, Red]}, 
   {0, Directive[Thick, Green]}, {1, Directive[Thick, Blue]}};

contours = DeleteDuplicatesBy[First]@
   Join[styledcontours, Thread[{automaticcontours, Automatic}]];

ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
 PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
 ColorFunction -> "Rainbow", ImageSize -> 500, 
 Contours -> contours]

automaticcontours = Cases[cp, Tooltip[_, t_] :> t, All]

{1.5, 1., 0.5, 0., -0.5, -1., -1.5};

styledcontours = Thread[{{0., -1., 1.}, 
     Thread[Directive[{Green, Red, Blue}, Thick, Opacity[1]]]}];

contours = Join[styledcontours , Complement[automaticcontours, {0., -1., 1.}]];

ContourPlot[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
 PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}}, 
 ColorFunction -> "Rainbow", ImageSize -> 500, 
 Contours -> contours]

ContourPlot

Hope one of the four methods below work in Version 7.0.

All the methods below add three styled curves, "while all other curves stay in the default style."

ContourPlot

1. Add constant functions in the first argument of Plot3D corresponding to the desired levels, set their PlotStyle to Opacity[0] and use the option BoundaryStyle to set the directives for the intersection of the main surface with the added planes:

2. Post-process to restyle selected mesh lines:

1. Use FindDivisions to generate a mesh list (that matches the automatically generated one) and add your list of styled mesh lines and use the combined list as the setting for Mesh:

automaticmeshlines = Most @ Rest @ FindDivisions[{-2, 2}, 18];

styledmeshlines = {{-1, Directive[Thick, Red]}, {0, 
    Directive[Thick, Green]}, {1, Directive[Thick, Blue]}};

mesh = DeleteDuplicatesBy[First]@
  Join[styledmeshlines, Thread[{automaticmeshlines , Automatic}]];

Plot3D[Sin[x - y] + Cos[x + y], {x, -10, 10}, {y, -10, 10}, 
 PlotPoints -> {30, 30}, PlotRange -> {{-10, 10}, {-10, 10}, {-3, 3}},
 ColorFunction -> "Rainbow", ImageSize -> 500, 
 Method -> {"RotationControl" -> "Globe"}, SphericalRegion -> True,
 MeshFunctions -> {#3 &},
 Mesh -> {mesh}]

2. Add constant functions in the first argument of Plot3D corresponding to the desired levels, set their PlotStyle to Opacity[0] and use the option BoundaryStyle to set the directives for the intersection of the main surface with the added planes:

3. Post-process to restyle selected mesh lines: