First, use our own predicate builder:

public static class PredicateBuilder
{
    public static System.Linq.Expressions.Expression<Func<T, bool>> Null<T>() { return null; }

    public static System.Linq.Expressions.Expression<Func<T, bool>> Or<T>(this System.Linq.Expressions.Expression<Func<T, bool>> expr1,
                                                        System.Linq.Expressions.Expression<Func<T, bool>> expr2)
    {
        if(expr1 == null)
        {
            return expr2;
        }
        ExpressionReplacer replacer = new ExpressionReplacer(CreateFromToReplaceSet(expr2.Parameters, expr1.Parameters), null, null, null, null);
        LambdaExpression rightExpression = (LambdaExpression)replacer.HandleExpression(expr2);
        
        return System.Linq.Expressions.Expression.Lambda<Func<T, bool>>
             (System.Linq.Expressions.Expression.OrElse(expr1.Body, rightExpression.Body), expr1.Parameters);
    }

    public static System.Linq.Expressions.Expression<Func<T, bool>> And<T>(this System.Linq.Expressions.Expression<Func<T, bool>> expr1,
                                                         System.Linq.Expressions.Expression<Func<T, bool>> expr2)
    {
        if(expr1 == null)
        {
            return expr2;
        }
        ExpressionReplacer replacer = new ExpressionReplacer(CreateFromToReplaceSet(expr2.Parameters, expr1.Parameters), null, null, null, null);
        LambdaExpression rightExpression = (LambdaExpression)replacer.HandleExpression(expr2);
        return System.Linq.Expressions.Expression.Lambda<Func<T, bool>>
             (System.Linq.Expressions.Expression.AndAlso(expr1.Body, rightExpression.Body), expr1.Parameters);
    }


    private static Dictionary<LinqExpression, LinqExpression> CreateFromToReplaceSet(IList<ParameterExpression> from,
                                        IList<ParameterExpression> to)
    {
        Dictionary<LinqExpression, LinqExpression> toReturn = new Dictionary<LinqExpression, LinqExpression>();
        for(int i = 0; i < from.Count; i++)
        {
            toReturn.Add(from[i], to[i]);
        }
        return toReturn;
    }
}

see this thread for more information and usage: http://www.llblgen.com/tinyforum/Messages.aspx?ThreadID=14144

This way, you can build dynamic predicates using Linq which are merged into the expression tree and which are used for the final query on the DB.